AN4077 datasheet(24/33 Pages) STMICROELECTRONICS

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High-voltage startup generator

AN4077

24/33

Doc ID 022997 Rev 1

As the VCC voltage reaches the startup threshold (12 V typ.) the low-voltage chip starts

operating and the HV generator is cut off by the VCC_OK signal asserted high. The device is

that it is made with an auxiliary winding in the transformer of the cascaded DC-DC converter

and a steering diode) develops a voltage high enough to sustain the operation. The residual

consumption of this circuit is just the one on the 15 M resistor (10 mW at 400 Vdc), typically

50-70 times lower, under the same conditions, as compared to a standard startup circuit

made with external dropping resistors.

At converter power-down the DC-DC converter loses regulation as soon as the input voltage

is so low that either peak current or maximum duty cycle limitation is tripped. VCC then drops

and stops IC activity as it falls below the UVLO threshold (9.5 V typ.). The VCC_OK signal is

de-asserted as the VCC voltage goes below a threshold VCCrestart located at about 6 V. The

HV generator can now restart. However, if Vin < VHVstart, HV_EN is de-asserted too and

the HV generator is disabled. This prevents converter restart attempts and ensures

monotonic output voltage decay at power-down in systems where brownout protection (see

the relevant section) is not used.

If the device detects a fault due to feedback failure, the internal VCCrestart is brought up to

over the VCCOff (turn-off threshold). As a result, shown in Figure 34, the voltage at pin VCC,

oscillates between its turn-on and turn-off thresholds until the HV bus is recycled and drops

below the startup threshold of the HV generator.

The high-voltage startup circuitry is capable of guaranteeing a safe behavior in the case of

short-circuit present on the DC-DC output when the VCC of both controllers are generated

by the same auxiliary winding.

Figure 39 shows how the PFC manages the VCC cycling and

the associated power transfer. At short-circuit the auxiliary circuit is no longer able to sustain

the VCC which starts dropping; on reaching its VCCOff threshold the IC stops switching,

reduces consumption and drops more until the VCCrestart threshold is tripped. Now, the

high-voltage startup generator restarts and when the VCC again crosses its turn-on

threshold, the IC starts switching. In this manner the power is transferred from mains to PFC

output only for a short time for each trep cycle.

Figure 34.

High-voltage startup behavior during latch-off protection

AM11465v1

(pin 13)

Vin

VccON

VccOFF

Vccrestart

VHVstart

HV generator is turned on

Disable latch is reset here

HV_EN

Fault occurs here

HV generator turn-on is disabled here

Input source is removed here

Vcc

(pin 14)

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