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Posted on 26 September 2019

Using Bidirectional Power Supply to Modulate Voltage on the Output

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Special technical requirements for switch mode power supplies are pushing to the limits of their performance, requiring a well optimized topology like the latest telecom RF power amplifiers, are well optimized.

By Milan Marjanovic and Roberto Scibilia, Texas Instruments

The key to increasing efficiency is envelope tracking or, “envelope of envelope tracking”, so-called slow drain modulation. In this case, the supply voltage will be modulated according to the required RF power, in order to minimize power dissipation on the output stage. Following on from the previous article, “High Speed, Two Quandrant DC/DC Power Supply describes the practical limitations of 350W telecom SDM power supplies.

Let us start with principles of the envelope tracking technique and slow drain modulation as shown in Figure 1.

Envelope tracking technique and slow drain modulation

The red line represents the envelope of the output RF signal. The blue line represents a voltage level of the system with constant voltage supply at the power amplifier. In this case the power losses are directly proportional to the length of this red arrow. Having the possibility to modulate the supply voltage (represented by the green line) accordingly, it is possible to reduce the average power losses dramatically.

This decreases the power consumption, thermal interface and its costs. All this is only possible using high speed, high dynamic Two Quadrant Power Supplies.

The repetition rate of this modulation can be in the range of several hundred Hz and the amplitude of the modulation can be greater than 12Vpp. If we take 500us as the maximum required for up slope and down slope, we can talk about 1kHz modulation. Tuning the voltage up and down periodically in no load condition, the converter will try to charge and discharge the capacitance on the output accordingly. Since we have a big capacitance at the output filter and big capacitance at RF amplifier as well, it is clear that the optimization of this parameter plays a key role. Usually the output filter is a LC filter. L has to be a storage choke (with low Resr and low core losses, a good rule of thumb is 25% ripple current) and an electrolytic capacitor big enough to place the frequency corner at two decades lower frequency than switching frequency. This is only the start point, the cross check with load step transient response has to be done as well. However, in this case we get the value of 160uF. If we calculate the power losses (caused by tanδ - low frequency losses in a capacitor) we will get the following:

equation 1

Considering that we have a 1kHz sine wave, the modulation amplitude is 12Vpp. If we have a triangular wave form these losses are going to be higher owing to high frequency harmonics. In addition, there are high frequency ripple current losses. Altogether, this results in very high losses for a small capacitor in a high temperature environment.

The alternative is to use ceramic capacitors. Ten or twenty such parts (standard 10uF/50V X7R capacitors) are needed to reach the required capacitance for filtering and sufficiently low impedance for transient response. The electrolytic capacitor has an apparently ‘magic’ behaviour with its low frequency zero: fz = 1/(2×π×Resr×C); i.e. 25kHz for this capacitor. This zero pushes up the phase of the power stage and changes the slope from -2 to -1, making possible loop compensation at higher cross-over frequency.

Using ceramic capacitor causes strong phase loss (red line), using simple resistors in series will pull the phase back (green line)

Using ceramic capacitors, there is very low RESR, pushing the fz to very high frequency (>250kHz). These force possible instabilities, since slope remains at -2, and push the practical cross-over frequency to lower frequency range, which is not of interest.

We can see these two situations in the simulated plots in Figure 2.

We can see that, only using ceramic capacitor causes strong phase loss and huge Q of the filter itself (high peaking of the resonant frequency, red line). Using simple resistors in series with the ceramic capacitor bank will pull the phase back and dampen the filter (green line). The new Resr value chosen is 50mOhm, which is close to the Resr of the electrolytic capacitor. This is why these two curves are close to each other. This value could be higher (giving more phase margin and better damping), but it is limited by reflected ripple voltage. Since the impedance of the capacitor bank is now minuscule versus external Resr, the complete measured ripple voltage on the output is now a measured voltage drop of the ripple current across this resistance. On the one hand the ripple voltage is given by the specification, and on the other, the ripple current is now limited by this external resistance. To make the right selection of these resistors we have to take care of the following power losses:

  • Ripple current losses (high frequency currents)- Pd_ripple
  • Modulation losses (low frequency losses)- Pmod_sine, Pmod_trian
  • Peak power losses (caused by huge load step current)- Ppeak

These power losses are calculated as follows:

power equations

Practical results

In figure 3 we see a top side view of a 350W SDM power supply for telecom applications- hardware.

350W SDM power supply, hardware top side view

In figure 4 a sine wave modulation is shown. Input control voltage is 1Vpp; output voltage is 12Vpp and modulating frequency is 1kHz.

Modulating voltage on the output, CH1 Output voltage 5V/div; 200us time base, CH2 Control voltage 500mV/div and CH4 Output load current 5A/div

In summary, the converter’s specifications are shown in the table 1.

Converter Specifications

Summary:

We discussed bi-directional power supplies (see also Article “High Speed, Two Quandrant DC/DC Power Supply”, “Voltage Mode Versus Current Mode”); using these to make high speed power supplies and how to modulate the output voltage. We discussed system setup, synchronous rectification, practical issues with regulation techniques and how to use it. In summary:

Use a topology with synchronous rectification for a high performance and high speed power supply. Only for this reason it is possible to work in two quadrants and therefore, achieve a good load transient behaviour.

Disable any burst or power saving mode and enable continuous conduction mode all the time. This reduces efficiency at low or no load conditions, but it is the only way to react quickly to high load current changes.

Avoid any components with low pass behaviour like optocouplers in the voltage feedback loop. The highest bandwidth is achieved, if the PWM controller is placed on the secondary side. Use voltage mode control and bi-directional current sensing to avoid instabilities when current flows backwards (from the secondary side to the primary side).

References:

“High Speed, Two Quandrant DC/DC Power Supply”, “Voltage Mode Versus Current Mode”

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