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Implementation of the primary-side regulation in flyback converters (Part 1 of 2)

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By eeNews Europe

Abstract

The consumer electronics market, and the LED drive market in particular, have been developing rapidly in recent years. These markets are demanding devices with lower power consumption in shrinking form factors. Today, many of those applications use flyback topology and secondary feedback control to adjust output regulation.

This method can’t reduce the number of components inside a device or reduce its size. Plus, it’s difficult to reduce costs. Addressing these challenges, this article will provide a new control method that can achieve power consumption savings, higher efficiency and low cost power supply: Primary Side Regulation (PSR)

Introduction

Most power supplies currently deployed in consumer electronics and LED drivers are based on the flyback converter architecture, which implements the traditional secondary feedback using phototransistors and error amplifiers in the secondary side circuit to achieve constant voltage and constant current (Figure 1). In this architecture, the main task of the secondary side circuit is to transfer the secondary side pulse signal to the primary side for the feedback loop to modulate the pulse duty-cycle. This stabilizes the output current and voltage applied on the load when the output load has undergone some change.


 

Figure 1: Flyback converter implementing traditional secondary-feedback control

Thus, this control method has penalties in terms of the number of components in the secondary side circuit, the increased PCB space occupation, and higher cost, as well as the power loss and increased standby power consumption caused by the required feedback signal test in the circuit. This article outlines the advantages of the Primary Side Regulation (PSR) circuit. The circuit regulates the output current and voltage applied on the load through the control within the primary side, relieving the burden on the secondary side feedback, Figure 2.

Specifically, the PSR circuit directly uses the voltage signal that it receives from an auxiliary winding on the transformer primary side to modulate the pulse duty-cycle, so as to stabilize the output current and voltage applied on the load.


 

Figure 2: Flyback converter implementing PSR control

Fundamentals

Traditional PSR controls the feedback signal by detecting the voltage (VDDZ) of an auxiliary power supply (Figure 3). In this implementation, the constant voltage is achieved by comparing the detected feedback voltage (VDDZ) with the operating voltage (VDD) on the controller that is proportional to the output voltage. As for the constant current, since it is in a non-continuous current mode flyback converter, the output power is proportional to the square of the primary side peak current.

It can be achieved by adding a compensation signal in the controller (Figure 4) and using the controller input voltage to regulate the compensation signal. However, the performance of this control method depends heavily on the quality of the coupling between the auxiliary winding and the secondary winding as well as the circuit design.


 

Figure 3: VDD feedback control circuit


 

Figure 4: Relation between VDD signal and compensation signal

As shown in Figure 2, the PSR control circuit outlined in this article uses the voltage signal that it receives from the auxiliary winding on the transformer primary side to regulate the control pulse as shown in Figure 5):

Figure 5: Time sequence of primary-side signal waves

1.     Once [ton] the MOSFET in the PSR controller is turned on, an input voltage [VIN] is established on the transformer and the primary side current [iP] rises from 0 to ipk. Since the energy from the input side is stored in the transformer during the MOSFET on-time, the peak current (ipk) can be calculated through following equation, where LP is the inductance of the primary winding; ton is the MOSFET on-time (Equation 1):

 

 During the MOSFET cut-off time [toff], the stored energy turns on the diode on the secondary side and is conducted to the load. And during this time, the output voltage and the forward break-over voltage of the diode will induce a voltage on the auxiliary winding [VAUX] that can be calculated through the following equation, where, NAUX /NS is the auxiliary/secondary winding ratio; VO is output voltage; VF is the forward break-over voltage of the secondary side output diode (Equation 2):


                

2.     Within the induction process, a sampling circuit in the PSR controller will detect a sample voltage from the auxiliary winding the VAUX, which, by Equation 2, provides information about the output voltage. The controller then compares this information with an internal reference voltage [VREF], and establishes a controlled MOSFET on-time to stabilize the output voltage accordingly.

3.         Due to the existence of the L-C circuit formed by the transformer inductance and the MOSFET output capacitor COSS, when the current in the output diode decreases to 0, the voltage on the auxiliary winding will oscillate, until the MOSFET turns on again. The discharge time constant [tdis] can be calculated from the sampling circuit, and as shown in Figure 3, the average current of the output diode is equal to the output current (IO), which can be calculated through the following equation, where, tS is the switching period of the PSR controller; NP/NS is the primary/secondary winding ratio; and RSENSE is the resistance of the sensor for secondary side current sampling (Equation 3):     .

 

Figure 6 shows a simple circuit implementing PSR control. In this circuit, the voltage on the auxiliary winding is detected through the Vs pin. This voltage is very close to output voltage when the secondary side diode is about to be cut off.

By comparing this voltage with an internal reference voltage in the controller, and by using the discharge time constant of the secondary side that the controller displays, voltage regulation can be achieved. The current regulation can be achieved by comparing the voltage on the detect resistor (RCS) with an internal reference voltage in the controller and using the discharge time constant of the secondary side that the controller displays.

However, this control method is not accurate. To enhance accuracy, we need to add a resistor and a capacitor in the VCOMV/ VCOMI detect loop.

 

Figure 6:  A simple circuit implementing PSR control

(End of Part 1. Part 2 will look at circuit design, and actual results)

About the authors

Sean Chen is a technical marketing engineer in the Asia Pacific region for Fairchild Semiconductor. He received his M.S. degree from Chung Yuan Christian University. Prior to his current position, Chen was a field application engineer for Fairchild in Taiwan. His research interests include AC/DC converters and the AC-DC converter market.

Eric Lan is the Vice President, Technical Marketing and Applications, Asia Pacific for Fairchild Semiconductor. He holds an M.S. Degree in Electrical Engineering, from the National Taiwan University, Taipei, Taiwan. He has a broad base of experience in power supplies, power-management ICs, power devices, magnetics, and EMI/EMC applications.


Lawrence Lin is a field application engineer for Fairchild in Taiwan. He graduated from the National Taiwan University of Science and Technology. His research interests are in high-efficiency and low standby-power consumption technologies of AC/DC power supplies.



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