Let us study a schematic diagram of a two-stroke half bridge converter, of an international naming «half bridge» (rice. 1).
While the gates of the transistors are not received voltage, they are closed. The voltage at the middle point of the capacitive divider, performed on the capacitors C1 and C2 of the same capacity, is half of the DC voltage, supply converter.
Will provide from the master oscillator to the gate trigger voltage VT2 of transistor. By chopping + Uent, capacitor C1, TV1 transformer winding, transistor VT2, -in theent potechet current. In the secondary winding of the transformer there TV1 voltage, which will be rectified diode VD1 assembly and smoothed capacitor C3. Transistor VT1 all this time has been closed.
Will provide a blocking voltage to the gate of the transistor VT2 and is based on the gate voltage of the transistor VT1. Current flows through the circuit + Uent,transistor VT1, TV1 transformer winding, capacitor C3, -in theent. In the secondary winding of the transformer will TV1 voltage of opposite polarity relative to the previous cycle, which straighten VD1 diode assembly and smooth out the capacitor C3. Then a DC voltage from the capacitor C3 is applied to the load. VT2 transistor during a second closed cycle.
How visible, current flows through the load during both cycles. output voltage ripple frequency of twice the frequency conversion, that allows the use of capacitor C3 the smoothing filter with a small rated capacity. Private hysteresis loop of the magnetic core of the transformer half-bridge converter close to the limit of the hysteresis loop.
As long as the load is not connected to the SMPS, each capacitor of the capacitive voltage divider is applied half of the DC voltage, supply converter. If the capacity of the capacitor voltage divider is not big enough, then at maximum load during each half cycle the capacitors are substantially discharged, and the voltage across them exceeds half the supply voltage converter.
Voltage, applied to the primary winding of the pulse transformer of the half bridge converter, It can be calculated by the formula:
in theus - saturation voltage of a transistor.
The capacity of each capacitor of the voltage divider can be calculated by the following formula:
Iperv.maks - the amplitude of the total current through the primary winding of a transformer;
F – frequency conversion, Hz;
ΔUwith - the change in voltage on the capacitor for the time duration of the total current passing therethrough pulse Iperv.maks.
The magnitude of the voltage applied to the capacitor of the variable component shall not exceed the maximum allowable reference value for a component of the brand and type of. Important to remember, the rated capacity of many capacitors at high frequencies and at low ambient temperatures is substantially reduced.
Half-bridge converters are widely used in power output from a few watts up to several kilowatts.
The virtue of the half bridge inverter is a low reverse voltage, attached to each transistor in the off state, approximately equal to the DC voltage converter power supply.
This allows you to use half-bridge converters with a high supply voltage. Half-bridge converters can be incorporated without load, and thus will not be dangerous damage to parts. Pulse frequency is twice the frequency conversion.
If the voltage divider capacitors are strictly identical, switching transistors are identical to each other, magntoprovoda and hysteresis loop of the material contains no defects, it can be assumed, that the magnetization of the core of the pulse transformer offline. Such a pattern can only be ideally. So, For example, real half-bridge converter, the capacitors in the voltage divider is always different from each other and, Consequently, nesymmetrychno peremahnychyvanye transformer. However, the degree of asymmetry is usually much less than the, than in the magnetic single-ended converters transformers. One of the simplest ways to reduce podmagnichivanmya core half bridge inverter is the inclusion of a non-polar capacitor between the pulse transformer and the midpoint of a capacitive voltage divider.
The disadvantages include the presence of two capacitors in a voltage divider, destruction of the components of SMPS overcurrent in the load in the absence of the protection system, lower efficiency, than attainable in the bridge converter.
A source: Power sources. Moskatov E.A.