Zero voltage drop synthetic rectifier

Transcription

1 Zero voltage drop synthetic rectifier Vratislav Michal Brno University of Technology, Dpt of Theoretical and Experimental Electrical Engineering Kolejní 4/2904, Brno Czech Republic In each AC powered system we often utilize one key part in the power supply chain rectifier. There are several ways how to design the rectifier. A common and well-used one is the full wave and full-wave center tapped Rectifier. According to the diodes used, there is always the voltage drop and consecutively power losses. This paper presents the design of the rectifier based on controlled MOS-switch, eliminates these voltage drop and power losses by use of small R DSON MOSFET parameter. Introduction Well-know full-wave bridge rectifier is shown in fig.1. Voltage drop results from the equation ev I = I 0 (exp 1) [1]. kt This voltage is for silicon diode aprox. 0.6V. The current passing through rectifier provokes the power looses: P loos =2*V F* I Load, c. 1.2W/A transformed to the parasitic heat. By using a rectifier with a higher current (approximately over two amperes), it is necessary to add a cooler, what should increase price and the place used. By using it with a smallest current, there should be an inconvenient in the voltages looses in the low-voltage applications applications. Fig. 1 - Full-wave rectifier By replacing the classical diodes by a controlled switch, we can reduce this looses to an insignificant level. For example, MOSFET unipolar transistors present the Ron resistance in scale closed to a few milliohms [1]. The idea is to watch the voltage between source and drain of the MOSFET and, by an additional circuit, to control the gate voltage. One of the principles is shown at fig.2. The inputs of the operational amplifier control the electrodes of the Fig. 2 Basic principle of synthetic diode realized by switched MOSFET low R DSON transistor

2 MOSFET transistor and by feedback affect his gate. When the voltage of Source is lower than voltage of Drain, comparator will shout down the voltage at the Gate and make the transistor conductive. Thereby only a small R DSON of transistor will be introduced in the current way. This property is the main advantage compared to silicon diode. Common used MOSFETs have a wide range of ON resistance and also thanks to the positive temperature factor allow to be connected in parallel and further to reduce the R DSON. We will call the new electrodes of synthetic diode, Synthetic Cathode and Synthetic Anode (sa, sc). This conception of diode is similar to Oring diode but the design has to be realized for both polarities with the integrated balancing unit (see below)[2]. Synthetic diodes conception This solution has a handicap in power supplying of the operational amplifier. The amplifier could be supplied from other voltage source, but it s unsuitable. For supplying this amplifier from the signal way, two conditions must be fulfilled: a) Amplifier has to be supplied by correct polarity b) Amplifier must have common input voltage range closed to the one polarity of supplying voltage. At a) Comparators supplying should be solved by connecting the power pin of the comparator at the synthetic cathode and second power pin at the second pole of voltage source. By this way, third new pin of the diode called supply S pin arise. Also we have to ensure the power supplying in the voltage rising, when the comparator is not powered and FET is non-conducting. Common power MOSFETs have in the internal structure the substrate directly connected to the Source, what forms the PN junction. By using this diode, we should cover the first wave of signal, but it will finally create the diode rectifier that we want to replace. Another solution is to connect the gate to the new third electrode via resistor and to keep the output of comparator in the state of high impedance during slope. Only the first moment, until the input voltage reach MOSFET threshold voltage, will be assured by diode. When the input voltage exceeds the Gate threshold voltage, transistor becomes conductive. When the comparator is supplied by sufficient voltage, his output assumes the control of the MOSFET Gate. Fig. 3 Example of input differential stage with PNP transistors and the output buffer At b) For the full-wave rectifier we have to design two diodes with different polarities of power supply pins. One for the positive supply switching with negative external supply pin (+D) and the second one for negative supply switching (-D) and positive supply pin.

3 Fig. 4 Synthetic components Positive synthetic diode (+D) and Negative synthetic diode (-D) Depending on polarity, the common input range of comparator has to reach up the positive (+D) or negative (-D) supply voltage. This property should be fulfilled by a proper realization of input differential stage (NPN for +D and PNP for -D). Example of NPN input buffer of differential amplifier which could operate in around of Vcc is shown in Fig. 3. When the MOSFET is controlled by comparator, the thresholds voltage increased by his saturation voltage. It means that we need an amplifier which is able to operate with input common voltage close to one polarity of supply and output voltage around second opposite polarity with minimal saturation drop. The reasons of these requirements appear on Fig..5. Fig 5 a) represents the final full wave rectifier and b) Full Wave full-wave Center Tapped Rectifier Circuit solutions of the diodes are shown at Fig.4. The R 2 resistor serves like protection. Fig. 5 - Full Wave and full-wave Center Tapped Rectifier realized by +D and D Measured Results Drop voltage The both prototypes (Full Wave and full-wave Center Tapped Rectifier) was realized. The obtained results respond with a consideration, that only a small R DSON is introduced into the signal way. The measurements were realized with a common transformer which presents a small distortion owing to saturation of the metallic core of the transformer. In fact this effect presents an advantage; indeed the ideal sinus output signal will enormously reduce the conduction angle of diode and will provoke an increase of the current peak. The distortion of transformer can reduce this current peak but nevertheless we have to consider higher current peak than the continuous current supplied into the charge. The Fig 6 shows the rectifier input versus output time characteristic for a) charged by 1A and b) 6A DC current (negative branch). One ampere DC current presents the insignificant voltage drop (in range of milivolts). With 6A, we can see the small voltage drop (c. 250mV) due to the RDSON (the

4 used one was 8mΩ) and to the construction resistances. Observable warming (+20 o C) has been measured with charge up to 15Ampers. Classical diodes rectifier (KBU8J) with this charge has to be cooled down from the charge current c. 2-3A. Fig. 6 Input versus output rectified measured signal (left 1A output current, right 6A output current) Strobe pulse In the Fig.7 the input sinusoidal signal and the gate impulse is shown. The fig a) shows no-loaded rectifier with narrow (1ms) strobe pulse and b) 6A loaded 5ms (π/2) long pulse. This relatively big conductive angle is due to internal resistance of transformer and also to a small filters capacitor (6mF) (we can see the input signal peak cut-down corresponding with control pulse) Fig. 7 MOSFET strobe pulse for non-charged and for 2A charged rectifier Strobe pulse overlap Like in the full wave rectifier, in the full-wave synthetic rectifier there is still two conductive diodes in series. (Fig. 1, Fig. 8). This couple of diodes forms the sub-structure in the full-wave rectifier, which will be important in the next design. Naturally the gate control signal for both polarities has to be exactly synchronous what can be corrupted by the internal structure asymmetry (different RDSON in +D and D, Different voltage offset of comparators etc.). Therefore the time deviation of the gate impulse was measured too. Fig 9 shows the time overlap of the +D and D MOSFET control pulse.

5 C Load Fig. 8 Pair of complementary synthetics diodes Left one shows the control pulse of non-charged rectifier and the right one with 2A charged rectifier. There is a good synchronism between both control signals. But there was also observed a small signal injection into transformer line due to the small time shift of these impulses and also to the Gate charge injection effect of MOSFETs. Fig. 9 MOSFET strobe pulse overlap for non-charged and 2A charged full wave rectifier Internal structure design For the main aim replacement of the classical bridge rectifier the one chip integration of mentioned structure has to be done. Ideal form is the two ports (one AD input, one DC output port). The topology corresponds with fig.8, where two serial complementary diodes are shown. There are two inputs differentials amplifiers as input stage. Q1-Q4 realized with PNP transistors with common input voltage range in around of 0V and Q7-Q10 NPN in around of Vmax. When the zero or maximum supply voltage exceeded power supply voltage, a base current of Q1 (Q7) appears. Resistors R3 and R4 decrease this current to insignificant level and protect the input transistor. Differential stage Q15, Q16 improves the symmetry. The output and inverted output buffers drive the Gate of power MOSFET. The most important parameters - Start-up voltage and line-in resistance depend of the MOSFETs used. By simulation with 2V threshold voltage MOSFET, the start-up voltage is around 2.6V. This structure has to be implemented twice for the full-wave rectifier realization.

6 Fig. 10 Transistor level design of synthetics diodes pair (see Fig. 8) The simulated start-up of the rectifier is shown for an ideal transformer with zero internal resistance and ideal sinus signal, and for a non ideal one (Rin =0,5 Ω) still with ideal sinus. We can see an increase of the conductive angle and a deformation of the peaks like in real measurement fig.7, 6. Rectifier was charged by small 1A current. Fig. 11 Simulated time characteristic of synthetics diodes - for an ideal transformer on the Left and on the right for a non ideal one (0.5Ω internal resistance). Upper characteristics are the input vs. output rectified voltage (see Fig.6) and lower ones are the MOSFET strobe pulse for both polarities. Conclusion: A rectifier is a widely used component in practically all kinds of equipments. By replacing the common rectifiers by switched synthetics rectifiers, we will reduce the losses in the system and avoid the use of the bulk and expensive coolers. For the expansion, it is necessary to realize this rectifier like an integrated component preferably with only one AC input port and one DC output port without an external power supply pins. Several prototypes of rectifier were realized and confirmed the good performances of this conception. It can be also used as a worst polarity protection. Implementation of this structure can be promising for massive production. Literature [1] Traité de L électronique, Paul HORWITZ, Winfield HILL, Cambridge University Press, [2] Aplication note AND8183