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Analysis of synchronous buck switching power supply computer - HC Network Electrical Industry

Introduction Synchronous buck switching power supply with output current, high efficiency advantages for battery-powered notebook computers and other products. Usually the various components of notebook computers, such as the CPU, chipset, graphics chip, memory, hard drives, optical drives, etc., on the power supply output voltage / current demand different. So many laptop internal power supply step-down DC converters to generate different output voltage / current power supply to meet the needs of different components.

Most of the synchronous buck converter will produce oscillations in the shutdown, the resulting negative pressure will often cause damage to certain components. This article explores the reasons for negative voltage and the corresponding countermeasures. problem description and theoretical analysis

Figure 1 is a synchronous buck power converter basic circuit, the power converter shutdown, will close Q1, Q2 turns on. Thus, inductance ground connection point and Q2, Vc is the output voltage, due to inductance, capacitance form a series oscillating circuit, the negative output voltage may exceed the allowed range of the load voltage.

Case of no load Assume synchronous buck power converter output voltage of 5V, and no load. Shutdown, the lower end of Figure 1 MOSFETQ2 turn, constitute a damping of the LC series oscillator circuit, Rs is the sum of the discharge path of resistance:

Rs = Rsense + Rinductor + ESR + Rdson (1)

Equivalent circuit obtained approximate solutions of quadratic equations are:

(2) By the equation (2) know that, when the inductor or capacitor is large, the oscillation frequency f smaller, smaller negative voltage amplitude. Therefore, to reduce the negative voltage, increase the value of inductance or capacitance. Theoretical calculations, consider only the damping caused by the loss of Rs did not take into account the inductor current is too large inductor saturation caused by the additional losses, therefore, the actual output negative voltage amplitude smaller.

Figure 3 is the design of the notebook computer power application of the more common dual output synchronous buck power converter MAX1631 shutdown (no load, output voltage 5V) output waveform, the negative voltage of-1.26V.

A load case Assuming Vc = 5V, load RL = 1.67 .

Equivalent circuit obtained approximate solutions of quadratic equations are:

(3) From equation (3) known, caused by the discharge path, the damping effect of RS than the load caused by the damping effect is also large, that is, the greater the Rs on reducing the amplitude of the more negative voltage to help.

Negative voltage ways to reduce 1) at the output of a Schottky diode in parallel to reduce the amplitude of negative voltage.

Figure 3 for the MAX1631 output parallel Schottky diode, the measured shutdown of the output waveform, waveforms can be seen from the Schottky diode can reduce the amplitude of negative voltage.

2) increase the damping resistance shutdown Rs.

Shutdown MOSFET through a low-resistance grounding of several ohms to 12W as an example, Rs = Rsense + Rinductor + ESR + Rdson = 12W.

Can see from the equation 3, to increase the damping resistance, capacitance and inductance to avoid oscillation, and its output waveform simplified RC discharge, effectively prevent the generation of negative voltages.

3) MAX1999 power converter. MAX1999 is Maxim's latest dual-output synchronous buck converter, which will shutdown the high-end and low end of MOSFET off, and through the 12 resistor at the output ground, the equivalent circuit into a simple RC discharge The output voltage equation

(4) MAX1999 in shutdown continuous monitoring of output voltage until the output voltage as low as 0.3V, the MOSFET turns on only the low end than the traditional shutdown mode with a negative voltage Schottky diodes suppress the effect of better and Schottky diodes can save the cost, is an ideal solution. The following is the MAX1999 shutdown

waveform analysis, assuming L = 7.6mH, C = 330mF, Rs = 12 , and Vc (t) = 0.3V, the output voltage from 5V down to 0.3V time is: Figure 4 shows the measured waveforms, which from 5V down to 0.3V the time 11.6ms, in good agreement with theoretical calculations, while the negative voltage swing of only 160mV, ensuring system safety and reliability. Conclusion

Traditional synchronous buck power converter in shutdown caused by a negative voltage components may cause system damage, several options discussed in this paper can effectively reduce the amplitude of negative voltage, in particular, to achieve optimal use of MAX1999 the effect on the use of Schottky diodes, a fundamental component to avoid the negative voltage causes the possibility of permanent damage, and can save system cost, is an ideal design.

Figure 1 synchronous buck power converter application circuit

Figure 2 MAX1631 in L = 8.2mH, C = 150mF, Vc = 5V shutdown waveforms

CH1: MAX1631's 5V output CH2: DL5 (low-side MOSFET gate)

Figure 3 MAX1631 in L = 8.2mH, C = 150mF, parallel Schottky output off when the wave plate and pipe

Figure 4 MAX1999 Shutdown Waveform

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