Onsemi Secondary Side Regulated Isolated Flyback Converter Instruction Manual

onsemi Secondary Side Regulated Isolated Flyback Converter

SPECIFICATION

DESCRIPTION

This evaluation board user’s manual provides elementary information about a secondary side regulated flyback converter NCV12711SSRGEVB built with the NCV12711 operated in current−mode control at 100 kHz. This control circuit offers many features to build an energy−efficient converter with all the needed protections like cycle−by−cycle current limit with a 250−mV sense voltage, over−current protection (OCP) and over−voltage protection (OVP) on the VCC pin. The controller drives an N−channel MOSFET as with any classical flyback converter at a user−adjustable switching frequency. The secondary side  hosts a low−Vf diode for efficient rectification in continuous conduction mode (CCM).
The primary−side section drives a transformer whose primary inductance is 8 H. The current is sensed via two paralleled 40−m resistors which limit the maximum output current to a safe value in fault condition. The board is rated to 12 W of continuous output power in free air at the lowest input voltage. This level is delivered down to a 4.5−V input. The converter is able to deliver output power up to 4−V input, which is the turn−off level adjusted by an UVLO resistor divider. At higher input voltages, the board may deliver more power but thermal runaway may happen and the board temperature must be monitored.
The regulation is ensured directly on secondary side requiring the use of an optocoupler. The advantage of this solution is better output voltage regulation in comparison with Primary−Side−Regulated converter.
The switch SW1 let you select different configurations to test the circuit:

1. a is closed, b open: in this mode, the VCC and VIN pin are connected together while the auxiliary winding is not used. The maximum input voltage is 25 V; going beyond this value will trip the OVP on VCC pin.
2. b is closed, a open: in this mode, the controller is supplied by the VIN pin only during start−up sequence and VCC is biased by the rectified auxiliary supply. The input voltage can go up to 45 V.
3. a and b are open: the controller is self−supplied via internal LDO and the auxiliary winding is not used. The input voltage can go up to 45 V.

Due to secondary side regulation, the switch SW1 affects only the efficiency of the system. For more details, see  the Efficiency and Standby data in TEST DATA section. The internal operational amplifier coupled to external components ensures the realization of a type 2 compensator. Using the simulation model or a bench measurement, components values were adjusted to crossover above 1 kHz.
The maximum crossover is limited by the right−half−plane−zero (RHPZ) which degrades  the phase response at the lowest input voltage and the largest output current. The board is equipped with two connectors letting you easily connect the network analyzers probes for a convenient measurement. The collected graphs show a comfortable phase margin at crossover.
A simple front−end filter limits the amount of parasitic noise going back to the source and it must be properly damped to avoid interaction with the downstream converter. C9 is providing that function with its equivalent series\ resistance (ESR).

KEY FEATURES OF NCV12711

• Internal 20−mA current source for lossless start−up sequence and self−supply operation
• Smooth start−up sequence with frequency sweep
• Internal operational amplifier with precise 2.5−V reference voltage
• Current−mode control operation
• Short circuit protection
• Over voltage protection
• Input Voltage UVLO with Hysteresis
• Shutdown threshold for external disable
• 0% duty ratio mode for low standby power
• Single Resistor Programmable Oscillator
• User−Adjustable Soft−Start Ramp
  

EVALUATION BOARD SCHEMATIC DIAGRAM

Figure 2. Schematic of the NCV12711SSRGEVB Evaluation Board

MAGNETICS DATA

ZA9654−AE from Coilcraft:

Figure 3. Mechanical Specifications

Figure 4. Electrical Specifications

TEST DATA

Startup Time

Steady−state Operation

Load Transient Response 

Output Voltage Ripple

Drain−Source Voltage

Standby Data

Table 1. NO−LOAD INPUT POWER WHEN THE IC IS SELF−SUPLIED VIA LDO

Table 2. NO−LOAD INPUT POWER WHEN THE VCC  PIN IS CONNECTED TO VIN PIN

Table 3. NO−LOAD INPUT POWER WHEN THE VCC PIN IS CONNECTED TO AUX WINDING

Efficiency Data

Table 4. BILL OF MATERIALS

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References

• Intelligent Power and Sensing Technologies | onsemi
• Intelligent Power and Sensing Technologies | onsemi
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• Intelligent Power and Sensing Technologies | onsemi
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