MAXREFDES120#: Isolated 24V to 5V–12.5W No-Opto Flyback Power Supply
Maxim’s power supply experts have designed and built a series of isolated, industrial power-supply reference designs. Each of these power supplies efficiently converts 24V into useful voltage rails at a variety of power levels. Each design has been tested for load and line regulation, as well as efficiency and transient performance. As with all Maxim reference designs, the BOM, schematics, layout files, and Gerber files are all available. In addition, boards are available for purchase; most boards feature through-hole pins for immediate board placement and accelerated prototyping.
MAXREFDES120# is an efficient no-opto flyback topology with 24V input, and a 5V output at 12.5W of power (2.5A). The design features the MAX17690 (a peak-current-mode converter with flexible switching frequency) and the MAX17606 (a secondary side MOSFET driver for flyback converter). The combination of no opto-coupler and a secondary side MOSFET result in a design that is optimized for space, cost, and efficiency. This entire circuit fits on a 20.3mm (0.8in) x 59.7mm (2.35in) board.
- Functional insulation
- Compact and flexible
- Minimal external components
- Robust operation in adverse industrial environments
- 5V output at 2.5A
- ±5% output accuracy
- Rapid prototyping
- Accelerated design time
- Cool system operation
- Industrial control and automation
- Process control
- Telecom and Datacom power supplies
Isolated power supplies provide reliable power regulation for a variety of applications. The MAXREFDES120# is a compact 24V input no-opto flyback converter module with secondary side synchronous MOSFET driver that features 5V at 2.5A output. Through-hole pins on the bottom side of the module allows quick and easy integration of this power converter into the main system. Pins are 15.24mm (0.6in) from power to ground, and 5mm (2in) from the input side to the output side.
The MAX17690 is a no-opto peak-current-mode controller for design of wide input-voltage flyback and boost regulators. The reference design operates over a 17V to 36V input voltage range, and provides up to 2.5A at 5V output. The switching frequency is programmable from 50kHz to 250kHz, allowing optimization of filter components resulting in compact and cost-effective power conversion. This design is set to switch at 100kHz. No opto-coupler or third winding are needed to derive a feedback signal across the isolation boundary since the MAX17690 senses the isolated output voltage directly from the primary-side flyback waveform during the off time of the primary switch.
The input undervoltage lockout (EN/UVLO) is provided for programming the input-supply start voltage (set to 17V in the design), and to ensure proper operation during brownout conditions. The EN/UVLO input is also used to turn on/off the IC. The overvoltage input (OVI) protection scheme is provided to make sure that the controller shuts down when the input supply exceeds its maximum allowed value (set to 36V in the design).
A capacitor is connected to the SS pin to program the soft-start period hence reduces the input inrush current during startup. Hiccup-mode overcurrent protection and thermal shutdown are provided to minimize dissipation in overcurrent and overtemperature fault conditions. On the secondary side, a MOSFET replaces the diode for a typical flyback converter. The MAX17606 drives the MOSFET. By replacing the secondary diode with a MOSFET, the MAX17606 improves the efficiency and reduces heat dissipation.
The reference design delivers a peak efficiency of 88.6% with the supplied components when the input is 24V. This general-purpose power solution can be used in many different types of power applications, such as programmable logic controllers (PLCs), industrial process controllers, industrial sensors, telecom power supplies, isolated battery chargers, servers, and embedded computing.
Figure 1. The MAXREFDES120# reference design block diagram.
Detailed Circuit Description
The MAXREFDES120# reference design contains all the control circuitry and electric components required for designing a 17V to 36V wide-input, 5V at 12.5W output isolated-flyback converter power supply.
When the primary side MOSFET is turned on, the current flows through the primary winding of the transformer and stores the energy. The CS-pin input and the current-sense resistor set the peak current limit and provide overcurrent protection. When the MOSFET is turned off, the energy stored in the primary side is transferred to the secondary side. The MAX17690 samples the output voltage by sensing the voltage of the transformer on the primary side.
The secondary side of the circuit consists of a MOSFET, a MAX17606 MOSFET driver, and a few output capacitors. The MOSFET emulates the diode in a typical flyback converter. The MAX17606 senses the voltage at the drain pin of the MOSFET to determine the timing of turning on/off the MOSFET. When the MOSFET on the primary side is off and the drain voltage of the MOSFET on the secondary side becomes less than -100mV, the MAX17606 turns on the MOSFET and the transformer pushes the current to the load at the output and the remaining energy is stored in the output capacitors. When the MOSFET is on and the drain voltage of the MOSFET on the secondary side becomes more than 30mA, the MAX17606 turns off the MOSFET to prevent the current flow back into the transformer and discharges the output capacitors.
The MAXREFDES120# system requires 2% of full load (about 50mA) for the output to regulate within limits. When the load is lower than 2%, the output is clamped by a 5.6V Zener diode.
Startup Voltage and Input Overvoltage Protection Setting (EN/UVLO, OVI)
The EN/UVLO pin in the MAX17690 serves as an enable/disable input, as well as an accurate programmable undervoltage lockout (UVLO) pin. The MAX17690 does not begin startup opera-tions unless the EN/UVLO pin voltage exceeds 1.21V (typ). The MAX17690 turns off if the EN/UVLO-pin voltage falls below 1.09V (typ). A resistor divider from the input DC bus to ground can be used to divide down and apply a fraction of the input DC voltage to the EN/ UVLO pin. The values of the resistor divider can be selected so that the EN/UVLO-pin voltage exceeds the 1.21V (typ) turn-on threshold at the desired input DC-bus voltage. The same resistor divider can be modified with an additional resistor, ROVI, to implement overvoltage input protection in addition to the EN/UVLO functionality. When the voltage at the OVI pin exceeds 1.21V (typ), the MAX17690 stops switching. Switching resumes with soft-start operation, only if the voltage at the OVI pin falls below 1.09V (typ). For the expected values of the startup DC-input voltage (VSTART) and overvoltage-input protection voltage (VOVI), the resistor values for the divider can be calculated as follows:
VSTART = (R2 + R3 + R4)/(R3 + R4) × 1.21 (V)
VOVI = (R2 + R3 + R4)/R4 × 1.21 (V)
If R2 = 274kΩ, R3 = 11.3kΩ, and R4 = 9.76kΩ, then:
VSTART = 16.95V, VOVI = 36.6V.
- MAXREFDES120# board
- One 24V DC power supply
- One electronic load
- 1 voltmeter
- 1 ammeter
The MAXREFDES120# board is fully assembled and tested. Use the following steps to verify board operation.
- Turn off the power supply.
- Connect the positive terminal of the power supply to the VIN pin of the MAXREFDES120# board.
- Connect the negative terminal of the power supply to the GND pin of the MAXREFDES120# board.
- Connect the VOUT connector of the MAXREFDES120# board to the positive terminal of the electronic load.
- Connect the negative terminal of the electronic load to the positive terminal of the ammeter.
- Connect the negative terminal of the ammeter to the GNDO connector of the MAXREFDES120# board.
- Connect the voltmeter across the VOUT and the GNDO connectors of the MAXREFDES120# board.
- Turn on the power supply.
- Set the electronic load to a constant current between 50mA to 2500mA.
- Verify the second voltmeter reading is 5V ±0.15V.
The MAXREFDES120# design was verified and tested under full input range and different output load conditions. The power efficiency vs. load current is illustrated in Figure 2.
Figure 2. Power efficiency vs. load current.
Figure 3 displays the output ripple at full load (ripple + spike is 180mVP-P) when the input is 24V.
Figure 3. Output ripple at 2.5A load, 5VOUT.
Figure 4 displays the load transient response when the load is stepped from 200mA to 1.2A, and then dropped back to 200mA again. The output transient spike is about 200mV. The input is 24V and output is 5V.
Figure 4. Transient response when load steps from 200mA to 1.2A.
Figure 5 shows the load transient response when the load is stepped from 1.5A to 2.5A, and then dropped back to 1.5A again. The transient voltage is about 200mV. The input is 24V, and the output is 5V.
Figure 5. Transient response when load steps from 1.5A to 2.5A.