Isolation power supply uses a boost controller MAX668 to generate three isolation voltages for the slice power supply_Learning

This application note shows a power supply that generates three isolation voltages for a slice. The isolation power supply uses a boost controller MAX668 with a flyback transformer for -24V and -72V outputs, and uses an optocoupler to isolate the feedback signal. The linear regulator MAX8867 provides a 3.3V output.

Some user line interface cards (SLICs) require power supply voltage to be isolated from local power sources. The circuit in Figure 1 generates three isolation voltages from a 5V input:+3.3V at 100mA, -24V at 100mA, and -72V at 25mA. It has a boost controller (U1) that operates in a transformer flyback topology and an optocoupler for isolating feedback signals. To provide this feedback (from -24V output to boost controller), the optocoupler (U4) is driven by a shunt regulator (U3) as an error amplifier.

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The SLIC power supply shown in Figure 1 uses a transformer to isolate three outputs (+3.3V, -24V, and -72V), and uses an optocoupler to isolate the feedback signal.

A custom transformer is required for a wide range of output voltage. Its core is a standard geometric coil SG3-0138 with a (L)=138nH/T ²。 The primary inductance is 6.8 µ H, and the peak current is 4A. The primary to secondary turn ratio is 1:5, so in order to achieve optimal efficiency, the duty cycle of the converter is close to 50%. The non regulated -72V output is derived from the regulated -24V output through three identical secondary windings connected in series. There is also a tap for -48V output, as well as a low-voltage winding that drives a linear regulator (U2) to provide+3.3V output. The winding specifications of the transformer are:

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At the maximum specified load, the circuit outputs 5.0V at 1.138A,+3.28V at 103.9mA, -24.0V at 100mA, and -73.2V at 25.2mA, with an efficiency of 80%.

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Note:

T1 wrapped around 138nH/T ² On the standard geometric core.

The primary inductance is 6.7 µ H.

Mainly on the 7th wheel and the 28th double track.

The secondary layer is three layers, with 35 times per layer and 32 times each time.

Low voltage secondary is 5 turns 32 #.

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