9.2.2.15 Estimating Bias Supply Requirements and C_VC

The bias supply (V_C) power requirements determine the C_VC sizing and frequency of hiccup during a fault. The first step is to determine the power and current requirements of the power supply control, then use this to select C_VC. The control current draw will be assumed constant with voltage to simplify the estimate, resulting in an approximate value.

First determine the switching MOSFET gate drive power.

1. Let V_QG be the gate voltage swing that the MOSFET Q_G is rated to (often 10 V).
   1. 
   2. Compute gate drive power if V_C is 12 V, Q_GATE is 17 nC, and Q_GAT2 is 8 nC.
      
   3. 
      P_DRIVE = 61.2 mW + 28.8 mW = 90 mW
   4. This illustrates why MOSFET Q_G should be an important consideration in selecting the switching MOSFETs.
2. Estimate the required bias current at some intermediate voltage during the C_VC discharge. For the TPS23754 device, 12 V provides a reasonable estimate. Add the operating bias current to the gate drive current.
   1. 
   2. I_TOTAL = I_DRIVE + I_OPERATING = 7.5 mA + 0.92 mA = 8.42 mA
3. Compute the required C_VC based on start-up within the typical soft-start period of 4 ms.
   1. 
   2. For this case, a standard 10-μF electrolytic plus a 0.47 μF should be sufficient.
4. Compute the initial time to start the converter when operating from PoE.
   1. Using a typical bootstrap current of 4 mA, compute the time to start-up.
   2. 
5. Compute the fault duty cycle and hiccup frequency:
   1. 
   2. 
      1. The optocoupler current is 0 mA because the output is in current limit.
      2. Also, it is assumed I_T2P is 0 mA.
   3. 
   4. 
6. With the TPS23754 device, the voltage rating of C_VC1 and C_VC2 should be 25 V minimum while with the TPS23756 rating can be 16 V.