This has been bothering me for a while.
In some of the design notes of power management ICs , it is mentioned that peak current mode control has 'lower gain - wider bandwidth' inner loop compared to that of average current mode control. Why is it so?
Quote from a design note-
"Peak current mode control has a low gain, wide bandwidth current loop which generally makes it unsuitable for a high performance power factor corrector since there is a significant error between the program signal and the current."
Thanks
In Power Factor Correction, average current mode control use the input voltage waveform as reference and the input current is usually continuous mode.
In peak current mode control, usually use discontinuous mode or boundary mode which have 'dead time' = zero current time. In boundary peak current mode control, the input current will be same waveform as input voltage in theory.
However, the dead time makes error of current waveform.
In the peak current mode control you are interested in control of fast transient current peaks. As is typical in a real current mode control IC, you have amplifiers that have real physical limitations. The internal error amplifiers have a finite maximum rise time. Current mode requires fast transient ability in order to preserve the integrity of the fast rising current waveforms. As with any operational amplifier you are trading off gain and bandwidth. The gain-bandwidth product is the controlling spec in this case. Having a wider bandwidth amplifier, you can get a clean and accurate output from the amplifier given a fast rising signal input, however, you must sacrifice some gain for speed. Conversely, you can decrease your bandwidth, and this will give more gain to work with. If your amplifier input signal is a slow rising signal, say a sine wave versus a square wave which has fast rising edges, or perhaps more proper, the signal does not have fast differential changes/edges, then you can apply higher gain without sacrificing signal integrity at the output. If you use too low of a bandwidth in an inner loop then your error will become larger as the amplifier output will not be able to rise, fall, or change quickly enough to output a clean waveform. It will distort. The current mode control scheme inner loop has fast rising signals and you need adequate bandwidth represent the signal accurately. Thinking of a Fourier Series, the more terms you use, the more sinusoids utilized to represent a waveform, the more close you get to the actual waveform. The higher frequencies add more and more fidelity as you add those terms. If you limit the number of terms too much, then you see the wave is less and less representative of the true shape of the waveform. Likewise, when you band-limit your amplifier your signal output representation will be less true to the actual waveform. In an outer control loop, such as the voltage control loop, the transients are slower compared to the inner loop. Reduced bandwidth is acceptable versus fast current transients. Amplifiers have a gain-bandwidth product that is constant. So you trade speed for gain, or gain for speed. For fast changing signals you need more bandwidth to accurately represent your signal. If you have a slow changing signal, then you may reduce bandwidth, which has the benefit to reduce noise, and you may then increase the gain. It's just a tradeoff that one must make when designing a system which has fast response as a requirement, and the decision you make regarding bandwidth and gain will affect the agility and accuracy of the system. Where accuracy/agility is needed, bandwidth must often be wide and this requires a sacrifice in gain. The pulse width modulation IC's are designed with proper bandwidth amplifier specifications, and also having sufficient gain to provide more precise output control. They generally suffice for most applications. The inner loop is faster, and needs more bandwidth, and I'd say that's actually the bottom line. I hope that this makes sense. If your load causes large current/voltage changes then you must keep that in mind when you select your PWM Integrated Circuit. So evaluating how your load changes with time may be critical to your choice of PWM IC so that stability of the system can be maintained, and output voltage as well as currents are well controlled, as is desired for your particular application. Average current mode results in less aggressive and accurate control of current, and therefore, it does not require as wide of a bandwidth and the amplifier gain can be larger, with increased phase margin and stability. Averaged current results in slower changing current waveforms - slower but stable. Faster but less stable. That's the trade we oftne have to make. I hope that this correctly answers the question for you, and did so in a way that makes sense.