Figure 1: Greatly simplified control block diagram shows two loops.
(Click on image to enlarge)

There are two feedback paths in the block diagram, one through an integrator where the output is compared to a reference and a second that compares the integrator output to the output voltage. The frequency responses of the blocks are shown in Figure 2. The blue curve represents the power stage response, which you may not have much flexibility to change. The load resistance is set by output voltage and current, and the filter capacitor is determined by noise requirements, switching frequency and transient load requirements.

You do have some control over the gain in both the optocoupler and current mode control portions of the power supply. The red curve is the response from the output voltage to the power stage input. With just an integrator for the compensation, you are somewhat limited in how you can compensate the power supply. At high frequencies, the gain from Vout to the power stage input is equal to one. The only choice you have is where to place the zero. This is determined by where the gain of the integrator goes to one. In Figure 2, the compensation zero is coincident with the power stage pole for an overall single pole roll-off. Note that since the compensation gain is one, the crossover frequency of the power supply is set by the 0 dB crossing of the power stage itself.

Figure 2: Connecting error amplifier as a Type-1 integrator limits bandwidth.
(Click on image to enlarge)

Many times, an integrator does not provide adequate bandwidth for the required transient response. An easy improvement is to turn the Type-1 error amplifier arrangement into a Type-2. Type-2 adds a resistor in series with the integrating capacitor and then adds a parallel high-frequency capacitor for a two-pole, one-zero frequency response.

Figure 3 shows the updated frequency response with a Type-2 amplifier. In this case, we are not restricted to 0 dB of gain at the first zero, and we’re able to set 10 dB of gain there. This allows the crossover frequency (where the sum of the two curves equals 0 dB) to be increased from 2 kHz to 6 kHz. Also, note the higher frequency characteristics. We placed a pole above the cross over frequency to reduce the noise sensitivity of the supply. And just like in the simple integrator, the gain though the compensation portion never falls below 0 dB.

Figure 3: Type-2 compensator improves bandwidth.
(Click on image to enlarge)

The higher crossover frequency enabled by the Type-2 error amplifier improves transient load response. Figure 4 shows the improvement with two designs having the frequency response characteristics shown Figures 2–3. The circuit was simulated in P-Spice and equal load steps were applied to both. As expected, a 3-to-1 improvement in bandwidth translates to a 3-to-1 reduction in the output voltage variation.                  

Figure 4: Type-2 Error Amp Yields a 3-to-1 Transient Step Load Improvement.
(Click on image to enlarge)

About the author

Robert Kollman is a Senior Applications Manager and Distinguished Member of Technical Staff at Texas Instruments. He has more than 30 years of experience in the power electronics business and has designed magnetics for power electronics ranging from sub-watt to sub-megawatt with operating frequencies into the megahertz range. Robert earned a BSEE from Texas A&M University, and a MSEE from Southern Methodist University.