AN2852 datasheet(7/35 Pages) STMICROELECTRONICS

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AN2852

Operating waveforms

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Operating waveforms

2.1

Asymmetrical half-bridge (AHB) typical waveforms

As mentioned before, this application note focuses on the AHB stage. This DC-DC converter

has the 400 V PFC bus as input and delivers 12 V at the output.

In Figure 3 the primary side key waveforms with full load applied are shown. Figure 4 shows

the detail of the two transitions during one switching cycle. When the LVG signal goes down,

the current is negative and so the half-bridge node (that has a certain capacitance value due

to the Coss of the MOSFETs and the stray capacitance of the circuit) is charged up to 400 V.

After the deadtime has elapsed the high-side driver is turned on with zero volts across the

high-side MOSFET drain-source pins. The driver activation is visible on the HVG signal

when there is the small voltage step on the high part of the waveform.

When the high-side driver is turned off, the primary current is positive, so the half-bridge

node is discharged down to zero volts and the body diode of Q4 is activated. After the

deadtime the LVG turns on in ZVS condition.

Figure 3.

AHB primary side key waveforms at full load

Typically, in the AHB topology, the most critical transition is the one between LVG turn-off

and HVG turn-on. In fact it is visible that the current available to move the half-bridge point is

less with respect to the other transition. This is due to the magnetizing current that is not

symmetrical with an average value of zero amps but has a certain offset due to the

asymmetrical driving of the tank circuit.

The fast current variation during transitions is due to the reversal of the current direction in

the secondary windings. The effort in this design was to maintain a negative current after

AM01818v1

Ch1: LVG pin voltage (yellow)

Ch3: HVG pin voltage (purple)

Ch4: primary winding current (green)

AN2852 Datasheet(PDF) 7 Page - STMicroelectronics