POWER SUPPLY DESIGN
Power factor correction
Milan Marjanovic & Matthias Ulmann look at how you can use a synchronous power rectifier with a microcontroller to manage voltage fluctuations
A large proportion of today’s research in the power supply sector centres around AC-DC power supply units. A modern power supply unit is expected to operate with a high degree of efficiency when connected to all power grids around the world. This requires a universal input (“wide input voltage range”), which covers the input voltage range of 85 V to 264 V. Such a high input dynamic range can only be compensated with power supplies cycled at a high-frequency. This also entails the advantage of making the power supply unit largely immune to voltage fluctuations at the input.
In principle all of these systems operate with
direct current. The input of each AC-DC power supply unit therefore features a rectifier, which uses diodes to generate direct current from the AC voltage (50/60 Hz). A bridge rectifier for single-phase systems consists of four rectifier diodes (referred to as a Graetz bridge). The diodes are interconnected in such a way that a fixed direction of flow of the output current is generated. In the majority of cases the rectified AC voltage is evened out even further by capacitors so that the output voltage exhibits a low ripple. Figure 1 shows the path of the current in the
bridge rectifier, the form of the input voltage, output voltage and current, and the mathematical relationships for the effective and average values. As the current follows the path of the voltage in an ideal system, the specified formulae are also valid for currents. Power losses are caused by non-ideal diode behaviour (Uforwards ≠ 0 and rdiode ≠ 0). They consist of static losses, which are caused by the forward voltage, and dynamic losses, which are caused by the dynamic resistance of the diode. The dynamic resistance can be calculated using two points from the linear region of the diode characteristic. U divided by I results in the dynamic resistance.
Equation 1
The first summand represents the static losses, the second represents the dynamic losses. The calculation model for multiple diodes connected in parallel is illustrated in Equation 2.
The following therefore applies to the bridge
rectifier: diode losses practically only result from static losses; a parallel connection of several diodes does not generate any real improvement and the static losses of the diode can only be reduced if another element that has a lower switch-on threshold than the diode and does not cause any voltage drop (or only results in a very minor voltage drop) is activated.
Equation 2 This equation can be further simplified:
Equation 3 The difference between Equation 1 and Equation 3 illustrates the influence of the parallel connection on the dynamic losses:
Figure 2: Diode & MOSFET parallel
Diode and transistor parallel connection If a transistor is connected in parallel to a diode, it will result in a circuit as illustrated in Figure 2.
Any modern transistor is suitable for this purpose. Equation 4
If, for example, ten diodes are connected in parallel, the dynamic losses correspond to only 10 percent of the losses of an individual diode. The static losses, however, remain the same. The minimum voltage drop, and thus the switch-on threshold, however, is still 0.7 V at 100 mA.
Example calculation for a half-wave • Mains voltage 230 V, 50 Hz, sinusoidal • Current amplitude 9.42 A peak a half-wave corresponds to 3.00 A DC or 3.33 A rms
The static losses can be calculated with Equation 5:
Equation 5
As the static losses only accrue in the transistor and not in the diode, it must be assumed that the diode does not conduct. This is the situation here, as the maximum voltage drop on the transistor is 386 mV. The minimum switch-on threshold of the diode, however, is 700 mV.
The power loss is recalculated with Equation 5:
The following therefore applies for the bridge
rectifier: this is only 16 percent of the losses compared to a bridge rectifier with four diodes and the switching and driver losses are negligible due to the low switching frequency of 50 Hz.
Texas Instruments has utilized this knowledge to develop a microcontroller-activated MOSFET
Figure 1: Current & voltage on the bridge rectifier 12 CIE Power Supplement April 2012
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