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SVRFL2800D - 10.0

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Page 8

SVRFL2800D Series

6.0 TECHNICAL NOTES

Please note that many of these

functions are also demonstrated

in detail on the VPT website in the form of technical video labs.

6.1 GENERAL INFORMATION

6.1.1 Topology Description

The SVRFL2800D Series is an isolated dual-output flyback converter. It provides a positive and negative output voltage with respect to

the OUTCOM pin. Up to 60% of the total output power is available from either output. The internal voltage regulation loop actively

regulates the p

ositive output using VPT’s proprietary magnetic feedback technology. The negative output is regulated by cross-

regulation of the transformer windings. The negative output is well-regulated for balanced load conditions. For unbalanced load

conditions, refer to the cross regulation performance graphs in Section 4.2 for expected performance. For a balanced or near-balanced

load condition, the converter will regulate down to zero load, and no minimum load is required. For an unbalanced load condition, with

negative loads greater than 10%, a minimum load of 10% is recommended on the positive output.

6.1.2 External Components

The SVRFL2800D Series is designed to operate stand-alone in most applications. It does not require any external components for

proper operation or to meet the datasheet specifications. Input and output L-C filters are provided internally for low ripple and noise. To

further reduce output ripple and noise, a small ceramic capacitor, 1 µF to 10 µF, can be added at the output. Most application specific

ripple requirements can be met with the addition of output capacitors alone. External output capacitance can be added up to the

maximum listed in Section 3.2.

6.1.3 Source Impedance

The impedance of the 28 V input source can interact with the DC-DC converter and can affect performance. High source impedance is

often caused by a long input cable or components added in series with the input. Source resistance will cause a DC voltage drop as the

converter draws DC input current. This voltage drop is simply the cable resistance multiplied by the input current at low line. The

voltage drop and the actual voltage at the input to the converter will determine the minimum source voltage at which the converter will

operate. A high source inductance can interact with the feedback control loop of the converter. VPT’s EMI filters will usually isolate the

source and eliminate this problem. In some cases, additional input capacitance will be needed to stabilize the system.

6.1.4 Output Configurations

Since the converter is isolated, the outputs can be used as a traditional dual-output, with a positive and negative voltage referenced to

OUTCOM, or as a single-ended output referenced to

–VOUT or +VOUT. For example, the SVRFL2812D model can provide +12 V and

-12 V in the traditional dual output configuration, or can provide +24 V referenced to

–VOUT, or -24 V referenced to +VOUT in single-

ended configuration. In the single-ended configuration, the OUTCOM pin will be at +12 V relative to

–VOUT.

The outputs of multiple converters can be stacked in series to provide higher voltages. When outputs of multiple modules are stacked,

they naturally share the load. For example, two SVRFL2812D converters can be stacked to provide a 48 V output at 200 W.

6.2 FUNCTION DESCRIPTIONS

6.2.1 On/Off Control (INH1 and INH2)

The INH1 (Inhibit 1) pin is a primary-side control pin referenced to IN COM.

The INH1 pin must be driven using an open collector or open drain

configuration. Pulling the INH1 pin low disables the converter output,

removes bias voltage from internal control circuitry, and puts the converter

in a state of minimum input current draw. Leaving INH1 open enables the

output, allowing the converter to operate normally. The pin must be pulled

below 1.5 V to disable the output. An optional capacitor from INH1 to

INCOM may be used to delay turn-on. The INH1 pin should be left open if

not used.