In the UCC256612 controller, the embedded HV pin supports start-up without X-cap discharge function. The HV pin connects to the DC bus directly (or through two resisters). The internal high voltage current source charges the VCCP pin during system start-up and turns the pin off when the VCCP equals VCCstartSelf (approximately 14V).

To support 800V start-up, the internal 640V rating of the UCC256612 is insufficient. Therefore, an additional high-voltage MOSFET stacked with the HV pin is required to implement high-voltage start-up. By employing a depletion-mode N-channel MOSFET, the system achieves a normally-on configuration, eliminating the need for external start-up bias resistors. We can choose BSS126 (600V depletion MOSFET with SOT-23 package) or BSS135 (same parameters as BSS126 with SOT-223 package) to extend the start-up voltage rating.

As shown in Figure 2-2, during the power-on stage, when VCCP is lower than VCC_StartSelf, the internal current source of HV pin pulls the source of BSS126 lower than its gate to turn BSS126 on. The internal high-voltage (HV) current source regulates the charging current of the VCCP capacitor ot a maximum of 10mA. When the VCCP triggers the VCC_StartSelf , the internal HV current source turns off and leaves the HV pin floating, and the source of BSS126 is higher than its gate to turn BSS126 off until the VCCP is discharged to VCC_{StopSwitching} (9.5V) to turn the HV pin on again.

To protect the system, we need to make sure the Vgs voltage of BSS126 is proper clamped. We have one resistor and a 12V Zener diode in parallel between Vgs of BSS126 to limit the voltage stress between 1V and -12V, which is lower than the maximum Vgs rating of BSS126 which is ±20V.

The next objective is to make sure the voltage stress on the V_DS of BSS126 and HV pin are sufficient. We use a pull-high resistor on the gate of BSS126 and clamp the voltage using a Zener diode. We used three 1206 resistors to support 600V voltage rating and four 130V Zener diodes which clamp the gate voltage to be approximately 520V. We choose approximately total 10MΩ pull-high resistors to lower the power consumption of the resistors.

Figure 2-2 Extend Start-Up Capability to 800V, V_G is the Measurement Point for the Start-Up Circuit

Before powering on the system for the first time, lower the input voltage to below the brown-in voltage to check the operation of the start-up circuit without the possibility of damage to the whole system. Figure 2-3 shows the start-up circuit working at V_IN = 50V. Here, check the VCCP and the V_G:

1. The timing of dip of V_G matches to the charging period in VCCP
2. The voltage of V_G is about half of the V_IN

We can incrementally increase the VIN to confirm the start-up circuit.

In practical design, the VG and HV pins exhibit very high impedance, making them highly susceptible to interference. Therefore, it is critical to maintain a quiet PCB layout around these pins by keeping high di/dt or dv/dt circuits away from the vicinity, thereby minimizing noise coupling.

Figure 2-3 Start-Up Waveform at V_IN = 50V CH1: V_IN, CH2: V_G, CH3: V5P, CH4: VCCP

Figure 2-4 shows the system start-up at 800V. It shows the V_G is clamped at 527V for about 100ms and then reduces to 427V which is around half of V_IN, which matches our design target.

Figure 2-4 Start-Up Waveform at V_IN = 800V, and V_OUT = 48V – 6.5A CH1: V_IN, CH2: V_G, CH3: VCCP, CH4: LO, CH5: V_OUT, CH6: I_RES