Onsemi Ncv12711psrgevb A 12-v 1-a Primary Side Regulated Isolated Flyback Converter User Manual

onsemi NCV12711PSRGEVB A 12-V 1-A Primary Side Regulated Isolated Flyback Converter

Product Information

• Product Name: NCV12711PSRGEVB
• Output Specification: 12 V/1 A
• Applications: Automotive
• Turn on Time: < 100 ms
• Input Voltage: 4 – 45 V dc
• Efficiency: Peaks to 88% @ full load
• Output Power: 12 W
• Operating Temperature:
• Topology: Current-Mode Flyback
• Board Size: 100 x 47 x 15 mm
• Cooling: Open Frame in Still Air
• Standby Power: See the tables on page 12

SPECIFICATION

Product Usage Instructions

The NCV12711PSRGEVB is a primary-side-regulated flyback converter designed for automotive applications. It operates in current-mode control at a frequency of 100 kHz. The converter offers various features for building an energy-efficient system with necessary protections such as cycle-by-cycle current limit, over-current protection (OCP), and over-voltage protection (OVP) on the VCC pin.

The controller drives an N-channel MOSFET, similar to a classical flyback converter, at a user-adjustable switching frequency. The secondary side of the converter includes a low-Vf diode for efficient rectification in continuous conduction mode (CCM).

The primary-side section drives a transformer with a primary inductance of 8 mH. The current is sensed using two paralleled 40-mW resistors, which limit the maximum output current to a safe value in fault conditions.

The converter can deliver a continuous output power of 12 W in free air at the lowest input voltage (4.5 V). It is capable of delivering output power up to 4 V input, but thermal runaway may occur, so it is important to monitor the board temperature at higher input voltages.

The regulation is achieved using an auxiliary winding, eliminating the need for an optocoupler. The auxiliary winding is filtered and rectified to provide a clean DC voltage. The circuit offers different configurations for testing:

1. a is closed, b open: In this mode, the VCC and VIN pins are connected together, and the auxiliary DC serves for regulation purposes only. The maximum input voltage is 25 V.
2. b is closed, an open: In this mode, the controller is supplied by the VIN pin during the 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: In this mode, the controller is self-supplied via an internal LDO, and the auxiliary winding serves for regulation purposes only. The input voltage can go up to 45 V.

In modes 1 and 3, which offer the best regulation figure, capacitor C12 maintains a constant load across it. However, in mode 1, the input voltage is limited to 25 V, while in mode 3, power dissipation may be a concern if a large-QG MOSFET is driven at a high switching frequency.

The NCV12711PSRGEVB features an internal operational amplifier coupled with external components to realize a type 2 compensator.

DESCRIPTION

This evaluation board user’s manual provides elementary information about a primary−side−regulated flyback converter NCV12711PSRGEVB 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 8H. The current is sensed via two paralleled 40−mresistors which limit the maximum output current to a safe value in fault conditions. 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 a 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 via an auxiliary winding, avoiding the use of an optocoupler. The winding is first filtered via R18/C19 and helps lower the leakage inductance peak naturally present in the transformer voltage. Then diode D4 with capacitor C12 provide adequate rectification to build a clean dc voltage. The switches 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 dc serves for regulation purposes only. 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 only serves for regulation purposes. The input voltage can go up to 45 V.

In the above three modes, 1. and 3. offer the best regulation figure because the load is constant across capacitor C12. However, in 1. the input voltage is limited to 25 V while in 3., power dissipation might be at stake if you drive a large−QG MOSFET at a high switching frequency.

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 analyzer 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

BOARD PICTURES

EVALUATION BOARD SCHEMATIC DIAGRAM

MAGNETICS DATA

ZA9654−AE from Coilcraft:

TEST DATA

Startup Time

Steady−state Operation

Load Transient Response

Output Voltage Ripple

Drain−Source Voltage

Loop gain Bode plots:

Regulation Data:

For all regulation measurements, the dummy load 1 k (R19) and Zener diode D5 were removed.

Efficiency Data

Standby Data

For all standby measurements, the dummy load 1 k (R19) and Zener diode D5 were removed.

Table 1. STANDBY INPUT POWER FOR Iout = 1 mA
WHEN THE IC IS SELF−SUPPLIED VIA LDO

Table 2. STANDBY INPUT POWER FOR Iout = 1 mA
WHEN THE VCC PIN IS CONNECTED TO VIN PIN

Table 3. STANDBY INPUT POWER FOR Iout = 1 mA
WHEN THE IC IS SELF−SUPPLIED VIA LDO

Table 4. BILL OF MATERIALS

• NOTE: Ceramic capacitors are X7R type unless stated otherwise.
• NOTE: TH = through-hole part.
• NOTE: All parts are lead-free.

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References

• Intelligent Power and Sensing Technologies | onsemi
• Intelligent Power and Sensing Technologies | onsemi
• Support
• Sales Offices & Distributor Network
• Intelligent Power and Sensing Technologies | onsemi
• Technical Documentation
• Support
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