Using adaptive compensation control to improve transient performance in power supplies with variable number of phases
The multiphase voltage regulator (VR), Figure 1, is the industry standard for providing regulated voltages and large currents for computing microprocessors. The multiphase power stage is designed to be efficient and thermally stable at a full loading condition, while its efficiency at light load is relatively poor in comparison to a VR with fewer running phases, Figure 2.
Figure 1: Multiphase voltage regulator block diagram
Figure 2: Efficiency with different phase number
The power consumption of microprocessors is highly dynamic depending on the application conditions, and the microprocessors may stay in low-power mode for long time. Therefore, it is important to achieve optimal efficiency over the whole load range. For Intel CPU Vcore power applications, the CPU can send out one PSI signal to the multiphase VR controller, which indicates that the CPU is running in a low-power condition. Once the VR controller recognizes that the CPU enters PSI mode, it can reduce the running phase number to reduce power losses for better efficiency in the low-power condition.
While variable-phase operation can improve overall efficiency of the VR, changing the number of active phases also impacts the transient response significantly. For small-signal analysis, the multiphase VR can be simplified as a single-phase VR with the output inductor equal to the phase inductor, divided by the phase number. When the number of active phases is reduced, the equivalent output filter changes.
This can result in a reduction of the overall bandwidth and phase margin of the VR. The reduced bandwidth can manifest in slower load-transient response and the need for more output capacitors to improve regulation. Therefore, the VR may have to be overdesigned to meet the transient requirements for a lower number of active phases, resulting in increased size and cost of the VR.
To solve this issue, PSICOMP technique can be adopted to change the compensation network in PSI mode. As shown in Figure 3, there is a PSICOMP pin to tie the extra R_PSI and C_PSI network to the feedback (FB) pin of the error amplifier, when the PWM controller closes the switch S1 in PSI mode.
Figure 3: PSI switch S1 to add the extra R-C network
The additional RC network can provide high-frequency gain and phase boost to help compensate for the changes in the LC filter due to the fewer number of operational phases, which can significantly improve stability and transient response.
As shown in Figure 4, the bandwidth of the one-phase VR is about 60 kHz, while three-phase VR can achieve over 100 kHz bandwidth with the same compensation network. With the addition of the extra RC network, the bandwidth of the one-phase VR is almost the same as that of the three-phase VR.
Figure 4: Loop gain comparison
As Figure 5 shows, there is over 50mV of undershoot and overshoot during the load-transient event with the original compensation network, which puts it "out of spec".
Figure 5: Transient response without PSICOMP feature
When the extra RC network is connected by the PSI signal, the transient response is improved, with only 20 mV undershoot and overshoot, Figure 6.
Figure 6: Transient response with PSICOMP
Note that this PSICOMP feature is introduced in Intersil’s latest VR12 controllers, including the ISL6364 and ISL6366.
About the authors
Weihong Qiu is a senior principal applications engineer in Intersil’s power management product group. He joined Intersil in 2002 and received a Ph.D. from the University of Central Florida in 2003.
Jason Houston joined Intersil in 2003 and is a staff applications engineer in the power management group. He received a BS degree from the University of Florida in 2003.
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