Fairchild puts current control into LLC PSUs

Fairchild has launched an isolated dc-dc controller chip which adds current-mode control to LLC resonant power supplies.

Called FAN7688, it has ‘charge current control’, instead of the voltage mode control traditionally used for LLC. “This “allows for tighter and faster output regulation against load transients and also simplifies the control loop,” said Farichild.

The technique involves combining a triangular waveform from the oscillator with the integrated switch current information to determine switching frequency, providing “a better control-to-output transfer function of the power stage simplifying the feedback loop design while allowing true input power limit capability”, said Fairchild.

Closed-loop soft-start prevents saturation of the error amplifier and allows monotonic rising of the output voltage regardless of load condition.

For improved efficiency, output rectification is synchronous using adaptive dead time control to reduce body diode conduction time – something branded ‘adaptive dual edge secondary side synchronous rectifier control’.

Efficiency for a for 400V in and 12V out power supply is “typically up to 97%”, according to a Fairchild FAN7688 video.

Operating frequency spans 39-690kHz, and the chip works over -40 to +125°C.

Protection includes over-load, over-current, output-short and over-voltage.

Applications are expected in servers, telecoms, industrial, PCs and TVs.

There are two evaluation boards: 306W FEBFAN7668 and 250W FEBFAN7688A.

LLC according to Fairchild:

The LLC resonant converter has been widely used becaus it can regulate the output over load variations with a relatively small variation of switching frequency.

It can achieve zero-voltage switching for the primary-side switches and zero-current switching for the secondary rectifiers over the entire operating range.

The resonant inductance can be integrated with the transformer into a single magnetic component.

Voltage-mode is typically used, where the error amplifier output voltage directly controls the switching frequency, However, compensation is relatively challenging since the frequency response with voltage control includes four poles and the locations of the poles changes with input voltage and load.

Charge current control is described in detail in the FAN7688 data sheet functional description section.

 

steve bush

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