PCPM | Peak Current-Mode DC-DC Converter Stability Analysis

Jun 1, 2010 12:00 PM
TIMOTHY HEGARTY Principal Engineer, Infrastructure Power Division National Semiconductor, Tucson, AZ

Furthermore, a smaller compensation capacitance is advantageous when the transconductance EA has a low output drive current capability.

DESIGN EXAMPLE

The circuit operating conditions, key component values and control circuit parameters of a buck converter based on one channel of a LM5642X dual-channel synchronous buck

The relevant gains and power stage corner frequencies are calculated using expressions (6) through (9) as follows:

The compensation component values, assuming a desired crossover frequency of 60kHz, are found using (29) and (30) as:

Figure 6 shows a Mathcad derived loop gain and phase plot of the exemplified converter. The equivalent plots with an ideal EA are also shown. The phase margin, ϕM, is the difference between the loop phase and -180° (EA inversion phase lag notwithstanding).

SIMPLIS SIMULATION

Using a LM5642X PWM controller in a buck converter configuration (Table), a SIMPLIS switching model circuit simulation is used to substantiate the analysis. The SIMPLIS model is presented in Figure 7.

The loop gain T(s) of the system is measured by breaking the loop at the upper feedback divider resistor, injecting a variable frequency oscillator signal, and analyzing the frequency response. The element with reference designator X1 is the SIMPLIS clock edge trigger to find the circuit periodic operating point (POP) before running the ac analysis. POP analysis works on the full non-linear switching time domain model of the circuit and enables subsequent ac or transient analyses. Figure 8 illustrates the bode plot simulation.

REFERENCES

1. R. Sheehan, National Semiconductor, ‘Current-Mode Modeling - Reference Guide’, www.national.com/analog/power/conference_paper_design_ideas

2. National Semiconductor LM5642 High Voltage, Dual Synchronous Buck Converter with Oscillator Synchronization from the PowerWise® Family, www.national.com/pf/LM/LM5642.html

3. R. B. Ridley, ‘A New, Continuous-Time Model for Current-Mode Control’, IEEE Transactions on Power Electronics, Vol. 6, No. 2, April 1991, pp. 271-280.

4. National Semiconductor LM3000 Dual Synchronous Emulated Current-Mode Controller from PowerWise® Family, www.national.com/pf/LM/LM3000.html.

5. National Semiconductor LM27402 High Peerformance Buck Controller with DCR Current Sensing from PowerWise® Family, www.national.com/pf/LM/LM27402.html.

Table: LM5642X Buck Converter Parameters

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