The HV Startup circuit utilizes two 600-V depletion mode MOSFETs (Q1, Q2). The depletion mode MOSFET conducts when no gate voltage is applied and begins to turn off as the V_GS voltage becomes more and more negative. It is completely off when V_GS is below the turn-off threshold. The characteristics of the depletion mode FET make it well suited to implementing a current source for high-voltage startup. It is difficult to find a low-cost and small-size depletion MOSFET with 1.2-kV rating, but there are wide variety of selection in 600-V to 800-V domain. Therefore, the stacked depletion MOSFET configuration with the proposed gate clamp circuit will evenly distribute the voltage stress from the 1-kV input voltage.

First, let’s look at the operation of Q1. Notice the four 130-V Zener diodes; D2, D4, D6 and D8. Their combined Zener voltage is 520 V. Next, think of R1 as a pull-up resistor to V_IN that simply provides current to the Zener diodes. With that in mind, it’s obvious that these diodes will be off if V_IN < 520 V. Now, as V_IN rises above 520 V, the voltage at the source of Q1 will be clamped slightly above 520 V, let’s say 521 V. In effect Q1 is biased such that the maximum voltage presented to Q2 is limited to 521 V. The V_DS voltage of Q1 is V_IN – 520 V. At 1000 V_IN, the V_DS of Q2 will be 521 V and V_DS of Q1 will be 479 V.

Next, let’s look at the operation of Q2. For now, let’s say D5 is a 22-V “safety” clamp to limit the maximum value of VDD in the event Q3 does not turn on. So, for normal operation it’s practical to assume D5 is off. When VDD < V_{DD_ON}, Q3 is also off because the controller has not been powered up yet and VREF = 0 V. R3 is a pull-up resistor (similar to R1 for Q1) that biases D9 on in the forward direction during HV startup. The majority of current flows from the source of Q2 through R5 and charges the 22-μF capacitor on VDD (C12). We can use KVL around the loop formed by R3, D9, and R5 and solve for the current through R5.

Typical values for V_{F_D9} and V_GS=V_{TH_Q2} are 0.3 V and 1.0 V, respectively. With this information we can solve for I_R5

Notice this current does not depend on V_IN so it will be constant over the entire range of V_IN.

If a soft start time requirement is provided (t_SS,MAX) the maximum value of R5 (R5_MAX) can be calculated.

Q3 functions as a simple switch controlled by VREF from the controller to shut down the HV startup circuit. When Q3 turns on zener diode D9 is reverse biased and clamps the V_GS voltage of Q2 to about ?18 V. Shutting down the HV startup circuit when it is not needed reduces power and improves efficiency.

This HV startup circuit is presented in detail and compared to traditional NPN-based HV startup circuit in “High-Density 40W Auxiliary Power Supply Utilizing a SiC MOSFET for 800-V Traction Inverters”, SLUAAL3.