Electronic Design

IGBT powers ultra low-cost offline wall adapter

Universal ac-to-dc wall adapters need to be light and compact to meet international traveler requirements. In that regard, switchedmode power supplies (SMPS), are the natural candidate for these applications. However, the total circuitry cost must be kept low to compete with common metallic transformer solutions. Figure 1’s schematic uses the new MMGN05N60D IGBT, recently introduced by Motorola, and represents a solution when space and cost are at premium. Associated with a dual-bipolar and a few surrounding components, the supply delivers up to 6 W with an efficiency of better than 70% at 120-V ac input.

At power-on, R3 charges C5 and the input capacitance of the IGBT (5-nC gate charge). When the threshold voltage is reached, M1 starts to close and applies the rectified dc rail across the primary. A voltage immediately appears on D1’s anode and accelerates the turn-on phase of the IGBT. This speed-up pulse allows a fast turn-on of the IGBT without degrading the overall efficiency.

The collector current rises with a slope of VIN/LP. Once the bipolar thyristor threshold is reached (IP = 0.7 V/R4), the gate is pulled to ground. The flyback transfer takes place, charging the output capacitor and elevating the output voltage. Regulation is obtained by offsetting the thyristor threshold and controlling the primary pulse width. We adopted a very inexpensive regulation network, but a complete square I/V battery charger can easily be implemented using the MC33341 in place of Zener D4. Figure 2 details the operating waveforms.

The supply is built with an off-the-shelf transformer featuring an LP of 6mH and respective auxiliary and power winding ratios of 0.2 and 0.06. It is important to keep the supply operating in discontinuous mode in order to cancel any turn-on losses. The MMGN05N60D has been carefully designed to keep the current tail losses at 6 µJ typically (@IC = 0.3 A, TJ = 125°C, and dVCE/dt = 1kV/µs), as shown in Figure 3. Furthermore, it includes a free-wheel diode that makes the device avalanche-proof. The MMGN05N60D represents a cost-effective solution versus a high-voltage MOSFET in off-line SMPS designs up to 6 W at 120 V ac.

One method of understanding the operation of this supply is to dive into the nitty-gritty of the system with the help of a Spice simulator. We simulated Figure 1’s schematic with Intu-Soft’s (San Pedro, Calif.) IsSpice4, and the results are gathered in Figure 4.

The simulation results agree well with the real measurements, highlighting the effects of the speed-up pulses. The time needed by the VGE voltage to reach the threshold level of 3.5 V is represented by t1. IC(t) ramps from 0 to IP, and its slope is defined by VIN/LP. the dead time corresponds to the thyristor reset and is constant at around 3 µs. Summing up these times yields the operating period. The IGBT Spice model can be obtained upon request by e-mail to the author.

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