Very High Voltage ICs, or VHVICs, make designing a power supply easy. Because of the monolithic solution, these devices minimize the number of external components and there is no need to sacrifice low-cost, size or efficiency to obtain additional features. VHVIC products are becoming more common and several switching power regulators are available. Applications include power supplies for office automation, consumer and industrial products and a wide variety of battery chargers. For example, a new series from Motorola contains monolithic high-voltage power switching regulators that combine the required converter functions with a unique programmable state controller. These devices are designed to operate directly from a rectified ac line source and are capable of providing an output power in excess of 90 W with a variable ac input that ranges from 85 V to 265 V. They can provide an output in excess of 150 W with a fixed ac input of 100 V, 115 V, or 230 V.

The series (the MC3337x) features a programmable state controller, an on-chip 700-V SENSEFET power switch, a 700-V active off-line startup FET, auto-restart logic, a fixed-frequency duty-cycle-controlled oscillator, a current-limiting comparator with leading-edge blanking, a latching pulse width modulator for double-pulse suppression, and a high-gain error amplifier with a band-gap reference for primary- or secondary-side regulation. The VHVIC process makes possible the combination of power-supply-specific circuitry and high-power switching devices in a monolithic structure. The process includes isolation of the high voltage applied to the power switch from lower voltage circuitry. Protective features include cycle-by-cycle current limiting, input under-voltage lockout with hysteresis, and thermal shutdown. These VHVIC devices are available in five pin TO-220 and DIP-8 packages.

A universal input 90-W off-line converter (see Fig. 1) is designed around the MC33374, the highest current-rated device in the series - switching peak currents up to 3.3 A (typical) and still be within current limit. This circuit requires fewer components than a power supply with a separate control IC and power switch to provide a 15-V, 6.0-A output from an 85 to 265 Vac input. Some of the features available in the VHVIC would require considerably more components to implement in a design with discrete components.

Fig. 1 A 90-W power supply built around the MC33374 VHVIC requires a minimal number of additional components and incorporates an on/off toggle. VHVIC process technology allows the monolithic combination of 700-V power devices with control circuitry.

Pin 1 (Fig. 1, again) is the positive supply voltage input. During startup power is supplied to this input from Pin 5. When V_cc reaches the UVLO upper threshold, the start-up MOSFET turns off and power is supplied from an auxiliary transformer winding. This pin may be connected to Pin 2, which is the feedback input - the shunt regulator error amplifier input - and is used to control the duty cycle of the power switch. It has an 8.6 V threshold and normally connects to the converter output, or to a voltage that represents the converter output. It is recommended (as shown in Fig. 1) that the feedback signal is provided by an opto-isolator.

This series has a single ground, pin 3, that serves as both a sense point for the shunt regulator/error amplifier and the high-current return path for the MOSFET power switch. Pin 3 is part of the integrated circuit’s leadframe and is electrically common to the metal heatsink tab. When a discrete power MOSFET is used in a power supply design, the tab of the TO-220 package is the drain and can be a potential antenna for common-mode emissions at high-switching frequencies. Since the series is designed in a VHVIC process, the ground is the tab for the TO-220 5-pin package, which minimizes the radiation problem. However, to ensure proper device operation and stability, it is important to minimize the lead length and the associated inductance of the ground pin. This pin must connect as directly as possible to the printed-circuit ground plane and should not be bent or offset by the board layout.

The power switch drain, pin 5, can be offset if additional layout creep distance is required. Due to the potentially high rate-of-change in switch current, components R3 and C5 must be connected to IC1 through separate, and short, copper traces. This will significantly reduce the level of switching noise that can be imposed upon the feedback control signal.

The state control block in the parts is accessed through pin 4. This block is designed to interface with a small number of external components to implement various methods of converter on/off control. By using the distinctive features of the state control circuit, the series can be programmed to enter into either the standby (no output from the converter) or operating mode in response to a variety of inputs. Inputs can come from a user-interface push-button switch, an optically-coupled microcontroller output signal, a combination of both, or other circuit configurations. The state control logic can be disabled and made to appear transparent when converter on/off control is not required.

In this circuit (Fig. 1, again) the on/off toggle switch implements a manual toggle. A toggle request is made each time the push-button switch is pressed and released. When the toggle comparator detects a request, its output clocks the state control logic, resulting in a converter mode change. Successive toggle requests cause the converter to alternate between the standby and operating modes. When in standby mode, the UVLO comparator and start-up MOSFET regulate the V_cc pin voltage in the hysteretic mode, and there is no voltage present at the converter output.

The 90-W supply has line regulation within ý24 mV with a Vin range from 92 Vac to 276 Vac and an output current of 6 A. The load regulation is within 26 mV at 115 Vac from 0.6 to 6.0 A and within 10 mV for a 230 Vac input over the same output current range. The output ripple is a total of 105 mVp-p over the 92 to 276 Vac range with an output current of 6.0 A. The efficiency with full output (6.0 A) at 115 Vac is 83.2% and at 230 Vac is 85.4%. The ac input power with the converter toggle off at 115 Vac is 0.07 W and at 230 Vac is 0.17 W.

L1 is a Coilcraft PCV—0—332—10, 3.3 mH, 0.005 W

T1, consists of:

Primary: 34 turns # 24 AWG; Pin 9 = start; Pin 6 = finish. Two layers 0.002" Mylar tape.

Secondary: 5 turns # 20 AWG, 2 strands bifilar wound; Pins 4/5 = start, Pins 1/2 = finish.

Two layers 0.002" Mylar tape.

Gap: 0.022" total for a primary inductance (Lp ) of 290 mH, with a primary to secondary leakage inductance of 7.2 mH.

Core: TDK PC40 EI28Z, PC40 material.

Bobbin: TDK BE—28—1110CPL, Pins 3 and 8 removed.

C8, C11 = Sanyo Os—Con #16SA1000M, 1000 mF/16V.

C12 = Sanyo Os—Con #10SA150M, 100 mF/16V.

IC1 = MC33374 mounted on Aavid #604953B02500 extruded heatsink.

Z1 = 1.5KE200A with cathode lead soldered in the center of a 5/8" x 3/4" x 0.025" thick U—shaped copper heatsink.

D7 = MBR20100 mounted on Aavid #590302B03600 heatsink.

Click here for the MC33370 data sheet.

A free guide from Motorola, "Switchmode Power Supplies Reference Manual and Design Guide" can be ordered at http://mot-sps.com/home/lit_ord.html or by e-mail at LDCFORMOTOROLA@HIBBERTCO.COM.

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