JAJSE40A October 2017 – February 2018 UCC28780
UCC28780 integrates two control concepts to benefit high-efficiency operation: peak current-mode control and burst ripple control. The peak current loop in AAM can be analyzed based on the linear control theory, so the compensation target is to obtain enough phase margin and gain margin for the given small-signal characteristic of an active clamp flyback converter. For transition-mode operation, the power stage can be modeled as a voltage-controlled current source charging an output capacitor (CO) with an equivalent-series resistance (RCo) and the output load (RO) as shown in Figure 37. The first-order plant characteristic and high switching frequency operation in AAM make the peak current loop easier to stabilize than ABM.
The adaptive burst mode (ABM) is a ripple-based control, so the linear control theory for AAM cannot be applied. The most critical stability criterion of burst control is to make the burst ripple content of IFB to be in-phase with the burst ripple voltage of VO. In normal operation, the fundamental burst frequency (fBUR) in ABM varies between 20 kHz and 40 kHz. An example of normal burst operation is illustrated in Figure 38.
Strong phase-delay in the frequency range creates slope distortion around the intersection point between IFB and ITH(FB), so the ripple regulator generates inconsistent burst off-times. As shown in Figure 39, the sub-harmonic oscillation at half of fBUR is a typical phenomenon of an unstable ABM loop. Two burst packets are adjacent to each other and the pulse count (NSW) is different by one pulse count.
In order to minimize the phase-delay of IFB, the transfer function from IFB to VO guides the pole/zero placement of the secondary-side passive ripple compensation network in Figure 40. In the primary-side control circuitry, two poles at ωFB and ωOPTO introduce phase-delay on IFB. ωFB pole is formed by the external filter capacitor CFB and the parallel resistance of the internal RFBI and the external current-limiting resistor (RFB). ωOPTO pole is formed by the parasitic capacitance of the optocoupler output (COPTO) and the series resistance of RFBI and RFB. For CFB = 100 pF, RFBI = 8 KΩ, and RFB = 20 KΩ, the delay effect of ωFB pole located at 278 kHz is negligible. However, ωOPTO pole is located less than 10 kHz, and introduces large phase delay in the interested fBUR range of ABM, since COPTO is in a few nF range contributed by the Miller effect of the collector-to-base capacitance of the BJT in the optocoupler output. Therefore, an RC network (RDIFF and CDIFF) in parallel with RBIAS1 is used to compensate the phase-delay of the optocoupler, which introduces an extra pole/zero pair located at ωP1 and ωZ1 respectively. The basic design guide is to place the ωZ1 zero close to the ωOPTO pole, and to place ωP1 pole away from highest fBUR.
Another guideline of obtaining a more consistent burst off-time is to maintain large enough ripple amplitude of IFB in ABM mode (ΔIFB) for better signal-to-noise ratio. Figure 41 shows that when the noise floor alters the intersection point of each burst cycle, larger ΔIFB performs much less burst off-time variation if the noise floor stays the same. ΔIFB around 10 μA is a recommended initial design value. The ripple ratio (KRIPPLE) between ΔIFB and the burst voltage ripple of VO in ABM (ΔVO(ABM)) is obtained by simplifying the small-signal gain of IFB(s)/VO(s) transfer function between 20 kHz and 40 kHz.
With the above understanding on burst control, the step-by-step design procedure is: