TI: Tips to design robust switching regulator systems for harsh automotive applications

By Texas Instruments
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Electronic control units (ECU’s) used in vehicle have to withstand harsh environmental conditions such as significant temperature variations, noise, vibration, transient supply voltages and transient load conditions due to sudden activation of loads/actuators such as motors and valves. This article gives an overview of things to look for while designing a switching regulator system for harsh automotive applications. Most of the examples are shown here are for BUCK regulators, but in general this can apply to boost regulators too. Read More


Introduction

Switching regulators are inevitable in most modern ECU’s used in automobiles to generate the power supply for different electronic components. Harsh environmental conditions, stringent quality and legal requirements can make the simplest switching regulator to fail, if it is not designed and verified correctly. When a switching regulator system fails, most of the cases we think that something is not correct with the regulator system control device (Integrated Circuit-IC). But a regulator IC can work fine only when the external circuitry and application conditions meet its requirements. Also like any other component, a regulator IC has its own tolerances and robust regulator system design should take such tolerances in to account and validation should catch any marginality in the design. 

Key component selections

Regulator IC/Device selection

Switching regulator IC with many topologies and features are available in the market. Depending on the application and cost requirements, devices with certain features and topology can be selected. With voltage mode control devices, pay special attention to compensation (due to its complex compensation scheme). In case of current mode control devices with more than 50% duty cycle, watch out for sub harmonic oscillations (due to insufficient slope compensation). Hysteretic Control devices have switching frequency variation and may need output capacitors with certain minimum ESR value. For regulators with internal compensation, often the range of LC product (output Inductor and Capacitor value) is specified in the datasheet. It is important to follow these recommendations as internally compensated regulators are optimized only for a small range of LC product. Safety related applications require special devices with certain safety features like independent monitoring of supply rails with independent bandgap and watchdog. Some devices have EMI performance related features like slew rate control and spread spectrum modulation. 

Pay attention to the device absolute maximum rating on each device pin and have sufficient margin to accommodate the supply variations and any transient overshoot/undershoot voltages. Absolute maximum ratings violations on input pin, SW/Phase pin and load current are very common root causes for IC damage.

Inductor selection

The output inductor is one of the most critical components in the switching regulator and it has major impact on the regulator performance. Follow the recommendations and equations provided in the datasheet to calculate the inductor value. If the inductor range is specified in the datasheet, often this value is effective inductance value considering its tolerances, temperature and load current.  Inductor series resistance (DCR) has an impact on the regulator efficiency and the inductor with lower DCR value should be selected for better efficiency. When choosing small chip inductors (like metal alloy or thin film), apart from current ratings, consider their voltage ratings too. In terms of Electro Magnetic Interference (EMI) performance, different inductors could show different performance and it is advisable to check the inductors EMI performance before finalizing it. Figure-1 shows how the inductor current will raise quickly when the inductor saturates in a typical buck regulator. Inductor saturation current rating should be more than the device current limit specifications.

 

Figure 1: Showing the saturated inductor current and non-saturated inductor current in a typical buck regulator

Even though the regulators have built in current limit feature, if the inductor saturates during its operation, during the current sense blanking time (very short time during which device switching MOSFET turns on to sense the current), the inductor current can quickly raise to a very high value and destroy the regulator IC. Also if the inductor saturates fully and loses its inductance, input voltage may appear on the output and this might damage the low voltage devices connected to the output of the regulator.

Input and output capacitor selection

Follow the datasheet recommendations for calculating the values of input/output capacitors. Depending on the temperature grade requirements, X5R or X7R type capacitors are used. For most of the automotive applications X7R capacitors are used due to its temperature characteristics. Typically regulator datasheets specify actual effective capacitance values and this value has to be calculated considering its tolerances, temperature and voltage derating. Capacitors have considerable variations across bias voltage and temperature and the standard capacitor value mentioned is assuming the DC bias voltage of 0V. For example 4.7uF / 16V rated capacitor could have only 20% of its rated value at 16V bias voltage. There are many online resources (example link:) available on this topic. Placing an additional small value capacitor (like 100nF) directly at the regulator supply pin may help to effectively filter the high frequency noise.

If the regulator needs to handle large load transients and needs tighter ripple voltage, it is good to choose the output capacitor which is 1.5 to 2 times larger than the calculated minimum output capacitor, but for the internally compensated regulators, strictly follow the datasheet recommendations.

 

Catch diode

For asynchronous buck regulators, catch diode selection is important in terms of efficiency and thermal performance. A Schottky diode with low voltage drop and low reverse leakage current should be used. In case of synchronous regulators, external Schottky diode may help to reduce the undershoot on SW/phase pin and thus provides better noise and EMI performance. During the blanking time (when both the transistors are turned off), current flows through the internal body diode of the low side transistor and this body diode has larger voltage drop and slower recovery compared to Schottky diode.

Pay attention to the high temperature reverse leakage current specification of the diode if regulator efficiency is a key parameter. Refer to the link for more details on this (example link: franks article). In case of synchronous regulators, adding an external diode might affect the current sensing of the low side transistor and the IC vendor should be contacted before using this diode.

Regulator functional modes of operations

Depending on the application needs, it is very important to define the regulator mode of operation. Some devices have automatic mode transition from low power mode to PWM mode depending on the load current and some devices have a dedicated mode pin to set the mode.

Low power mode operation:  Some devices have dedicated low power mode where special techniques (like shutting down most of the internal circuits) are used to optimize the regulator efficiency. Such regulators have good efficiency (more than 80%) during low power mode and often a dedicated pin to switch between low power mode and PWM mode is available or they may have auto transition capability from one mode to another mode depending on the load current.

Some devices just use pulse skipping method during light load condition to reduce the switching and gate drive losses to improve the efficiency, but these devices usually have poor efficiency at light load condition (around 50%). If the application requires high efficiency during low power mode operation, such devices may not be ideal. Also regulator input current can vary significantly depending on many external factors such as output inductor and capacitor.

In low power mode, output ripple will be significantly higher. The regulator switching behavior mostly depends on the load current and often has large dropouts followed by very narrow switching pulses. During the large dropouts, Inductor inrush current can be significantly high and the inductor could saturation if inductor is not selected correctly. Narrow switching pulses can cause large undershoot/overshoot on SW pin due to parasitic inductances in the switching path. Also since the switching frequency is not clearly defined during low power mode operation, EMI performance is not predictable and filtering becomes difficult. Figure 2 shows an overshoot seen on the SW pin during low power mode transient operation causing a violation of the absolute maximum rating of the SW pin at cold temperature. At cold temperature, the device switches faster, external parasitic inductances dominate and the input capacitor performance degrades. Figure 3 shows a typical low power mode operation inductor current and switching behavior. Even though the input average current is less than 10mA, actual peak inductor current is around 5A!

 

Figure 2 : Overshoot (10V) seen on SW pin during low power mode transient operation causing violation of absolute maximum rating of SW pin at cold temperature

 

Figure 3: typical low power mode inductor current seen

It is advisable to avoid low power mode of operation unless it is absolute necessary. For ECU’s which are active even when the engine is not running (for example: E-CALL ECU), low power mode cannot be avoided. In such cases detailed measurements are necessary to cover all the possible transient system behaviors. If regulator operating current is measured in low power mode during the ECU production test, test limits should be based on measurements done on larger set of samples and limits should be wide enough to accommodate larger variations.

Fixed frequency mode (PWM Mode)

In this mode, the regulator is always switching with fixed frequency and depending on the load current it may be in continuous conduction mode (CCM) or discontinuous conduction mode (DCM). DCM suffers from low efficiency and higher noise and higher ripple voltage. This may not be good for EMI. However this mode of operation occurs only at light load for a given inductor value and by adjusting the inductor value it is possible to adjust the load current at which the device enters DCM. However synchronous regulators mostly work in CCM mode of operation irrespective of the load current. Figure 4 shows a gain/phase plot taken in DCM and CCM mode and in case of DCM mode, the regulator shows significant degradation in phase margin.

However in the case of boost, buck-boost, and fly back converters, DCM mode has some advantages in terms of loop compensation as DCM mode does not have a right half plane zero and it can have higher loop bandwidth to achieve better transient response.

 

Figure 4: gain/phase plots showing CCM (phase margin > 60 degree) and DCM (phase margin < 45 degree) mode of operation

Do not operate the buck regulator in DCM mode. It is important to know the regulator load current in your application.

100% duty cycle mode or low dropout mode

When the input voltage comes close to output voltage, some regulators offer 100% duty mode where the high side transistor is turned on for more than 1 clock cycle to maintain the output regulation at lower input voltage. Most of the regulators work on the principle of boot strap capacitor gate drive to drive the high side transistor. It can be turned on as long as there is sufficient charge on this boot strap capacitor. To recharge the capacitor, high side transistor needs be turned off and low side transistor needs be turned on. Therefore, regulator ripple performance will be affected and also there is considerable power loss across the high side transistor if the gate drive is not strong enough to turn it on fully. On some asynchronous regulators high side transistor will be turned off if the boot strap capacitor voltage drops below a certain threshold. During light load conditions, there is not enough energy in the inductor to pull the SW/phase pin to pull down to recharge the boot capacitor. This causes output voltage to drop until the boot capacitor is recharged above its threshold to turn on the high side transistor again. During this mode, some devices operate with a low frequency switching to recharge the boot capacitor. Disadvantage is that the switching frequency is not predictable anymore and the regulator performance degrades.

100% duty cycle mode is not intended during normal operating mode and this mode is useful only during some transient conditions. Hence it is recommended to have sufficient headroom on the input voltage to avoid this mode of operation.

Parallel operations of two switching regulators

Some applications may need two switching regulator outputs connected together to increase the load current capability. Such operation requires special care as matching of components of the two regulators is very important and it will have practical implementation difficulties.  In some cases two regulator output are tied together to enable redundant operation and this may have some issues related to back feeding when one of the regulator is not working.

Do not attempt to operate the regulators in parallel unless it is recommended in the datasheet.

Loop stability and load transient measurements

Loop stability analysis is very critical for any switching regulator and it will help you to understand the regulator behavior well. If the equipment is available, measurement itself is very quick. But if it is necessary to tune the compensation components to improve the regulator performance, some additional effort and time is needed. This is something engineers should not overlook as it can easily cause production fallout/field failures if not optimized well. It is one of the most frequent issues faced with switching regulators. Since it is a closed loop system level concern, IC manufacturers cannot give any guarantee on the regulator stability, but detailed guidelines and equations are always provided in the datasheet for designing stable regulator.

Due to parasitic effects or system level interactions, sometimes loop stability measurements may not give a full understanding of the regulator behavior. Hence it is also necessary to do the time domain load transient analysis at system level to see whether the regulator is stable for large load transients or not. Figure 5 shows the gain/phase plot taken on a customer ECU and on an Evaluation Module (EVM) for the same circuit. Loop response is different and the large load transients showed oscillation on the customer ECU. Figure 6 and Figure 7 shows the load transient behavior before and after optimization of the compensation. Figure 8 and Figure 9 shows the effect of adding a feedforward capacitor in the feedback path. It can be seen that with large CFF the bandwidth increased but the phase and gain margin is reduced significantly which can result in stability issues.

Useful tips regarding the loop stability

  • Follow the datasheet guidelines and equations while selecting the inductor, output capacitors and compensation components
  • Do not place any filter components in the feedback path unless recommended in the datasheet.
  • Simulations can be used as reference, but measurements have to be done on the board as PCB layout can have impact on the regulator stability and pay attention to PCB layout design
  • Phase margin of more than 45 degree and gain margin of more than 10 dB is considered to be stable. Loop stability measurement is a linear small signal analysis and large signal behavior has to be verified using load transient measurements.
  • Perform the load transient measurements with minimum to maximum load current conditions and make sure that the regulator does not show any ringing behavior. If the regulator shows ringing or it takes a long time to settle during or after the load transient, this needs to be addressed and the loop has to be optimized.
  • Do not use the feedforward capacitors unless recommended in the datasheet or recommended by the IC vendor. Also large feedforward capacitors can cause stability issues. Keep the feedforward capacitor to the minimum value which gives the stable response.

 

 

Figure 5: Showing the effect of PCB and external components on small signal loop measurements

Figure 6: Load transient test showing the unstable behaviour for the loop characteristics shown in Figure 5

Figure 7: Load transient test showing the stable behaviour after optimizing the compensation components
 

Figure 8 : Effect of 10pF feedforward capacitor on one boost regulator

 

Figure 9: effect of large feedforward capacitor on a buck regulator

Line transient, device start-up/shutdown behavior and regulator soft start

Many regulators have minimum on /off time requirements and hence the input voltage range at which they work with guaranteed full performance is limited. Some regulators offer 100% duty cycle operation mode, but with certain degraded functionality as described previously. Under certain transient conditions, it is possible that the regulator input voltage can come close to its output voltage or even drop below its output voltage. Or the input voltage can go high (violation of minimum on time) and all these transients may result in strange regulator behavior. Some regulators can have issues with the slew rate of the input voltage as internal circuit may need a certain time to respond to the sudden changes of the supply voltage. During start-up and shutdown events, overshoot/undershoot on Vin, SW/Phase or Vout pins can happen. During start-up events, large inductor inrush current can cause excessive power dissipation in the device. Most regulators have soft start functions to control the inductor inrush current and output voltage ramp up. Soft-start is a very important feature to protect the device against uncontrolled inrush current. Depending on the LC values used, soft-start time needs to be adjusted. Some devices have internal soft-start and such devices usually support only a limited range of LC values and datasheet guidelines should be strictly followed. Figure 10, Figure 11 and Figure 12 shows examples of different regulator behaviours during line transient conditions when the input voltage falls below output voltage and then comes up again without soft start.

 

Figure 10: Inductor inrush current seen with one regulator when Vin drops below Vout and comes back again without device reset/soft-start

Figure 11: Vout overshoot seen on a regulator when Vin drops below Vout and comes back again without device reset/soft-start

 

Figure 12: Vout overshoot seen on a regulator when Vin drops below Vout and comes back again without device reset/soft-start

It is always recommended to reset the regulator (with EN pin control) when Vin comes very close to Vout. This ensures that the output voltage comes back again with a proper soft-start. If the regulator is not reset, it is possible that device does not do the soft start and this results in uncontrolled inductor inrush current and Vout overshoot. Also check the startup and shutdown behavior of the regulator for possible overshoot or inductor inrush current which may violate the device specifications.

 

Thermal considerations and mission profile

TI device datasheets which are published in recent years have detailed thermal parameter table based on thermal simulation and this data is according to JESD51 standard. Thermal Metrics application report SPRA953 provides more details on how to use TI’s thermal parameter table and how to do the correct measurements. It is very important to understand which parameter from the thermal table must be used for junction temperature estimation. Thermal performance heavily depends on the application environment, PCB and power pad connection to the PCB thermal pad. It is always recommended to do the measurements on the ECU instead of theoretical calculations.

The datasheet maximum operating temperature rating does not mean that the device is guaranteed to work at maximum operating temperature for the whole of its life. If the device is classified as Q100, grade-1, means that manufacturer guarantees High Temperature Operating Life (HTOL) operation with an ambient temperature of 125C for 1000 Hr (or 150C for 400 Hrs). Based on the mission profile, if the device life exceeds these numbers, usually manufactures do not give any guarantee.

Most switching regulator devices have thermal shutdown features for self-protection when the junction temperature exceeds a certain specified junction temperature. This feature is just a secondary protection and in real application conditions, care should be taken not to get into thermal shutdown condition. Usually the thermal shutdown temperature exceeds the device maximum rated junction temperature and IC vendors cannot guarantee functionality beyond their electrical specifications under all application conditions.

The thermal performance of the device is heavily dependent on PCB design and for more accurate thermal estimation real measurements have to be done on application boards. If the device is working in high temperature applications, based on the application mission profile, it is recommended to get the estimation of device operating life from device manufacturers. Do not depend on thermal shutdown as a protection mechanism and keep the device junction temperature below the datasheet recommended operating temperature.

Layout considerations

Layout is a critical portion of a good power supply design. Several signals paths conduct fast changing currents or voltages that can interact with stray inductances or parasitic capacitances and generate noise or degrade the power supply performance. It is always good practice to do the PCB parasitic extraction of main power path and important signals. For example, parasitic inductance from input capacitor to supply pin, parasitic capacitance from SW plane to GND pin, parasitic resistance and inductance of the GND plane are very important. There are many PCB parasitic extraction software tools are available in the market.

Every TI datasheet provides layout recommendations and layout diagram. It is advisable to strictly follow the layout guidelines provided in the datasheet and layout improvements based on PCB parasitic extraction is useful.

Measurement setup

It is very important to have the correct measurement setup for testing the switching regulators. Voltage ripple should be measured directly across the output capacitor with a high bandwidth differential probe or using the walking stick method of probing. High frequency undershoot or overshoot on device pins should be measured directly at the device pin with a high bandwidth probe with a shortest GND connection using walking stick method. To do transient measurements like inrush current measurements, load and line transient measurements, a good input power supply is needed. Using the real automotive battery is recommended for optimization of circuit which has direct influence of the input supply, for example, optimization of inrush current during start up. Electronic loads often have pulsed currents or offer different impedance characteristics and hence for ripple measurements, load transient measurements and stability measurements, it is always advisable to use a purely resistive load. Figure 13 and Figure 14 shows how oscilloscope measurement techniques can affect the results significantly

Figure 13: Showing ripple measurement with different probe options                 

                      

Figure 14: Showing good and bad SW signal undershoot measurements

It is very important to have the correct measurement setup for testing the switching regulators and incorrect measurements may lead to wrong conclusions and a bad design. Measurements across different ambient temperatures are necessary as switching regulator system performance can vary significantly across temperature.

Conclusion

All the basic aspects of standard switching regulator systems are discussed along with real use case examples. Proper component selection, using the right mode of operation and understanding the regulator functionality under different operating conditions are prerequisite for robust switching regulator system design. Perform the detailed validation on the EVM with actual external components to be used in the ECU. Optimize the regulator performance on the EVM and make sure that regulator meets all the application requirements before finalizing the ECU circuit diagram. Pay attention to the PCB design and perform the detailed ECU level validation.    

 

About the author

Krishnamurthy Hegde was born in Karnataka, India in 1979. He has completed his bachelor’s degree in Electronics & Communication Engineering and Master’s Degree in VLSI & System Design. He has more than 15 years of experience in electronics industry. Before joining TI India in 2011, he had worked for 7 years in ECU development for automotive braking applications. In TI India he worked for 3.5 years as characterization and test engineer for high speed, multi-channel ADC’s and AFE’s for medical and automotive applications. In 2015, he moved to TI Germany as Applications Engineer for automotive switching regulator devices in TI’s Mixed Signal Automotive team. He is responsible for application support of automotive single channel switching regulator devices and power management devices (PMIC) for ADAS applications.

 


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