Step-up DC/DC voltage converter, 12/300 volts. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Voltage converters, rectifiers, inverters

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Almost every SMPS includes a full-wave rectifier that converts an alternating voltage of 220 V to a direct 310 V (the only exception is low-power SMPS, in which a half-wave rectifier is sometimes used). This means that to power such SMPS there is no need to generate a sinusoidal voltage of 220 V and a frequency of 50 Hz, but a constant voltage of 310 V is sufficient, which significantly simplifies the design of the converter.

The proposed DC/DC converter allows you to power any network electrical equipment with a power consumption of no more than 12 W, which includes an SMPS, from the vehicle’s on-board network or other 50 V DC voltage source. The converter has small dimensions and weight, high reliability and efficiency at low cost and simple design. Disadvantages - lack of galvanic isolation of the 310 V DC output voltage circuit from a 12 V power source and low power. The converter circuit is shown in Fig. 1.

(click to enlarge)

Specifications:

• input voltage, V.....10,5...15
• conversion frequency, kHz.....40
• output voltage, V ...300...310
• current consumed in the absence of load, mA ..... 38
• maximum long-term load power, W.....50
• Efficiency (at input voltage 11,7 V and load power 32 W),%.....92

The device is built according to the classical scheme of a push-pull converter with the output of the midpoint of the primary winding of the step-up transformer T1. The basis of the device is a push-pull SHI controller DA1. the output of which is connected according to the emitter follower circuit. The conversion frequency is about 40 kHz, it is set by resistor R3 and capacitor C3. Smooth start of the converter is ensured by elements R4, R7, C9, VT7. This protects fuse link FU1, switching transistors VT5, VT6 and rectifier diodes VD7-VD10 from overload during the transient process when smoothing capacitors C18-C20 are charging. When the supply voltage is applied, capacitor C9 is charged, transistor VT7 is closed at this moment.

As the capacitor charges, transistor VT7 opens and the voltage at the input of the “dead time” comparator (pin 4DA1) decreases. Due to this, the duty cycle of the controller pulses smoothly increases from zero to the maximum value (48%). This solution, in contrast to the commonly used RC circuit, makes it possible to obtain the maximum duty cycle of control pulses due to the extremely low drain-source resistance of the VT7 transistor in the open state. Diode VD2 accelerates the discharge of capacitor C9 when the supply voltage is turned off.

Transistors VT1, VT2, as well as VT3, VT4 are emitter followers that provide fast recharging of the gate capacitance of field-effect transistors VT5, VT6. Diodes VD3, VD4 bypass resistors R8, R9 in the gate circuits, speeding up the closing process of these transistors, thereby reducing switching losses. To limit voltage surges at the drains of transistors VT5, VT6 to a safe value, limiting diodes VD5, VD6 are installed.

To stabilize the output voltage, voltage feedback is applied to the DA1 error signal amplifier built into the SHI controller. The output voltage of the converter is supplied through a resistive divider R14R15 to the non-inverting input of this amplifier (pin 1 of DA1). The inverting input of the amplifier (pin 2) through resistor R1 receives voltage from the built-in reference voltage source (5 V) from pin 14 of DA1. An increase in the output voltage leads to a linear decrease in the pulse duration at pins 9 and 10 of the SHI controller DA1, which leads to a decrease in the output voltage, i.e. its stabilization.

Using resistors R1 and R2, the gain of the built-in error signal amplifier is set to approximately ten. This made it possible to prevent a significant difference in the duration of control pulses at pins 9 and 10 of the PHI controller.

On elements DA2, HL1, R10-R13 there is a battery discharge monitoring unit. The supply voltage from the divider R10, R11 is supplied to the control input of the DA2 microcircuit - a parallel voltage stabilizer, which is used as a comparator

When the voltage at the control input is more than 2,5 V, a current flows through the HL1 LED, the glow of which indicates the normal voltage of the battery, and the absence of light indicates its discharge. Diode VD1 protects the device from incorrect polarity of the supply voltage - if such a situation occurs, fuse-link FU1 burns out. The supply voltage to the SHI controller DA1 is supplied through the power filter L1C4C6.

The device uses resistors MLT, C2-23, oxide capacitors are imported, capacitors C1-C3 are K10-17, the rest are ceramic for surface mounting of standard size 0805 or 1206. Transistors IRF3205 are interchangeable with IRFI3205 or IRL3705N. transistors 2SC3205 and 2SA1273 - on KT961 and KT639, respectively (with any letter indices). In the latter case, it is advisable to select specimens with a static current gain of at least 100. Diodes 1.5KE36A can be replaced with diodes 1.5KE39A, 1.5KE47A or P6KE36A, P6KE39A, P6KE47A, and UF2007 - with FR207 or HER207. Full analogues of the TL494CLP PHI controller are microcircuits. KA7500 and KR1114EU4. After installing transistors VT5 and VT6 on the board, a common heat sink is attached to them through insulating heat-conducting gaskets. which is an aluminum plate with dimensions of 50x20 mm and a thickness of 2...4 mm. For transformer T1, a W-shaped magnetic core of El type with a cross-section of 10x7 mm from the IBM PC AT power supply is used.

First of all, winding II is wound onto the frame, containing 182 turns of PEV-2 0,25 mm wire, each layer is insulated with tracing paper. For winding I, five twisted wires PEV-2 0,44 mm are used; it contains 14 turns with a tap from the middle.

After winding, the entire reel is impregnated with shellac. To speed up its drying, you can warm up the coil by passing a direct current of 0,3...0,4 A through winding II. At this moment, there should be no magnetic circuit in the coil. To obtain maximum winding inductance, both parts of the magnetic circuit are glued together with shellac in which ferrite powder is mixed. After drying, the magnetic circuit is wrapped in several layers of paper adhesive tape. the inductance of each half of winding I of transformer T1 must be at least 130 μH.

All elements of the converter, with the exception of the LED, diode VD1, fuse holders, power switch, input and output sockets, are installed on a printed circuit board made of one-sided foil fiberglass 1,5 mm thick, the drawing of which is shown in Fig. 2.

The board is installed in a case measuring 154x64x39 mm - it is homemade and glued together from 2 mm thick polystyrene sheets. The LED, fuse holders, power switch, input and output sockets are installed in holes on the side walls of the case (Fig. 3).

Diode VD1 is at the terminals of the power switch SA1 and the fuse holder FU1. There are ventilation holes in the housing cover (Fig. 4).

Setting up the converter comes down to checking the output voltage, without a connected load, which should be in the range of 300...310 V. If necessary, it is changed by selecting resistor R15.

To set up a battery discharge control unit, it is necessary to select resistor R11 so that when the supply voltage decreases to 10,8 V, the HL1 LED goes out.

Author: Belyaev S.

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