Question list
1: Current sampling and overcurrent protection settings related issues
2: Auxiliary power supply buck voltage related issues
Problem Description
1. Current sampling and overcurrent protection setting problem
A. Product design parameters
Vin=36-75V, V0=12V, P0=400W (Imax=33A)
B. Corresponding schematic parameters are as follows
1. CS_NEG resistance to ground: R926=680ohms
2. Resistance between CS_POS and current sampling point CS: R925=680ohms
3. CS_SET setting resistance: R927=20Kohms
(The above corresponding overcurrent point is 39A, Vin high and low voltage difference is 3A; when R927=18kohms, the overcurrent point is 42A)
4. SSSR secondary soft start setting capacitor: C909=0.22uF
C: The problem to be solved is as follows
(1). Problem one : Synchronous rectification duty cycle rise process V0 power down problem
The C909 setting will affect the overcurrent protection snoring output, and affect the light load output voltage of Io<4A as shown in Table 1.
C909=0.1uF, light load (when the output is loaded with <4A), the output voltage is powered down (Figure 1, Io=2A)
Analysis：
The above problem has been analyzed. If the SSSR pin capacitance is set too small, the light load output voltage Vo of Io<4A will be powered down. The reason is that the synchronous rectifier is switched from diode rectification to synchronous rectification, and the power stage filter inductor current is off. DCM enters CCM which consumes the energy of the output. To solve this problem, the SSSRG pin capacitor C909 needs to be increased (C909>0.22uF), but it contradicts the problem 2 below.----How to solve the problem of Vo power failure during the rising cycle of synchronous rectification?
chart 1 is to reduce the output voltage dropout of C909 to 0.1uF under light load conditions.(Yellow: Synchronous rectification soft start SSSR voltage，Blue: output voltage)
(2). Question two: how to control overcurrent protection snoring time
The C909 setting affects the overcurrent protection snoring output and affects the Io<4A light load output voltage (see Table 1).
C909=0.22uF, the light load output voltage is normal, and the output voltage time is fixed at 80-100ms (Figure 2).
C909=1uF, the output voltage time can be shortened to 30ms (Figure 3), and then continue to lower C909, the output voltage time has not changed significantly, further analysis, the over-current protection snoring Vo duration and C909 (SSSR capacitance) rise The time is consistent (Figure 4)
C901 (RES Snoring time period setting capacitor) needs to start charging after the SSSR capacitor is fully charged (combined with Figure 4 and Figure 5)
Analysis：
During the overcurrent process, the RES pin response is related to the SSSR. The RES timing time follows the SSSR rise to the reference voltage time. By increasing the RES pin capacitance C901, the snoring has no duty cycle output time, but the duty cycle output cannot be effectively changed. time
The overcurrent protection snoring cycle is related to SSSR. Is it still related to SS? If the SSSR capacitor is too large, the Vo oscillation time will be lengthened, resulting in MOS heating, voltage and current surge, and the reduction (no change under 0.01uF, oscillation time 30ms) will occur when the output starts to be powered down. How to control the duration of Vo during overcurrent? According to the datasheet description, the duty cycle is only related to the RES value. How should the value be used?----How to solve the problem that the output voltage Vo of the snoring process lasts too long?
(Figure 2 shows the change of the snoring time after adjusting the C901 RES capacitance without changing the output voltage duration.)---yellow is output voltage
(Figure 3 is to adjust the C901 SSSR capacitor 0.01uF, the output voltage duration is reduced to 30ms)---yellow is output voltage
（Table 1 Overcurrent parameter setting and phenomenon relationship）
Figure 4 shows the relationship between C909 (SSSR capacitor) charging and output voltage Vo.---yellow is C909 voltage，blue is Vo
Figure 5 shows the relationship between C901 (RES capacitor) voltage and output voltage Vo.---yellow is C901 voltage ，bule is Vo
(3). Question three:How to interpret the output voltage clamp when overcurrent protection
During the overcurrent protection process, the output voltage Vo during the snoring process first rises to 10V and drops to 8V (combined with Figure 4 and Figure 5), and does not rise to the output voltage setting of 12V, and cannot analyze why it rises to 10V and falls to 8V，Why does Vo fall directly to 8V (hold) and then turn off after rising to 10V, instead of going directly to 12V and then shutting down? So what is the reason for rising to 10V and dropping to 8V?----How to explain the phenomenon that the output voltage of the snoring process is clamped at 10V/8V?
(4). Question four：about CS settings
a. Why is the overcurrent protection Io value the lowest when the input voltage Vin is the highest (the highest overcurrent protection Io value is the highest, and the nominal value is 4A)
b. Need to carefully explain the design of the current sampling / overcurrent protection part of the specification, and explain the role of the output POS_OCP and NES_OCP in Figure 27 of the datasheet.
(5). Question five：Auxiliary power supply BUCK voltage related issues
Need to solve the problem as follows
The setting of the auxiliary power reference voltage FB_AUX is related to the SS capacitor. Why is it that when Vss<2V is 1.4V and Vss>2V is 1.0V, what is the point?
