Power Tips: When to choose multiphase


There are many applications that can benefit from a multiphase power supply.  Some examples include core supplies for ASICs or Processors, car audio supplies or memory applications for servers.  Virtually any power supply can take advantage of a multiphase approach.  The advantages of a multiphase supply can include thermal performance, size, output ripple and transient response.  This approach can be used for simple buck converters, boost converters or even more complicated designs such as active clamp forwards or flyback converters.

Thermal performance related to conduction losses of the power supply is proportional to the current squared.  If multiple phases are used these losses can be reduced.  If two phases are used, power due to conduction losses is cut in half.

  • Single phase conduction losses = 
  • Two phase conduction losses = 

  • Four phase conduction losses = 

The conduction losses are only a portion of the total losses in a power supply, but at higher currents these losses can be significant.

Reduce power supply size by going to a multiphase approach.  Even though more components are required, the sizes of these components are generally smaller.  The magnetics can take up a large portion of the power supply footprint, even though more pieces would be required, the overall volume could be reduced.  The size issues are not just relevant to really high current supplies; sometimes lower current designs can also benefit and shrink in size.

Perhaps one of the strongest benefits of the multiphase approach is ripple current cancellation.  This ripple current cancellation helps out with input and output capacitors.  The figures shown below are an example of how ripple current cancellation can reduce the rms current in the input or output capacitors.

Multiphase power supplies figure 1: buck input caps, boost output caps

Figure 1:   Buck Output Caps, Boost Input Caps

 

 When to choose multiphase power supplies figure 2: buck output caps, boost input caps

Figure 2: Buck Input Caps, Boost Output Caps 

The figures are normalized and show how the rms current is reduced based on the number of phases and duty cycle. Figure 1 is for input capacitors in a buck converter or output caps in a boost converter.  Figure 2 is for output capacitors in a buck converter or input caps in a boost converter.

Multiple power stages can be used to improve the transient response of power supplies.  The main reason for the improvement is the ability to reduce the inductance of the magnetics to allow current to build up faster.  The smaller magnetics make for larger ripple currents, but because of the ripple current cancellations, the ripple performance can be kept constant.  The smaller magnetics can also help with increasing the bandwidth of the converter.

To illustrate the advantages of a multiphase converter, consider these power supply specifications:

  • Vin = 12V
  • Vout = 1V
  • Iout = 40A
  • Output Ripple Frequency = 500KHz

Single phase versus two phase design

 

Single Phase Design

Two Phase Design

Conduction Losses

P

½ P

Switching Losses (maintain 500KHz)

P

½ P

RMS Input Ripple Current

0.3*I

0.2*I

RMS Output Ripple Current

0.9*I

0.8*I

Number of Components

X

1.5*X

Inductor Volume

17mm x 17mm x 7mm

2023mm3

2 x 10mm x 10mm x 4mm

800mm3

 

To conclude, multiphase power supplies offer many advantages over a single phase approach.  Thermal performance, input and output ripple current, size and transient response can all improve using a multiphase approach, the only disadvantage being a slightly more complicated design with more components than a traditional single phase approach. The good news is we make multiphase power supplies easy with the following tested PowerLab designs using TI’s controllers designed specifically for multiphase design.

Multiphase Buck Converter power supply designs:

  • PMP2277 – Three Phase 60A Synchronous Supply using the TPS40180
  • PMP3054 – Four Phase 80A Synchronous Supply using the TPS40140
  • PMP5621 – Four Phase 80A Synchronous Supply using the TPS40140 and CSD87350
  • PMP7328 – Two Phase 60A Synchronous PMBUS  Supply using the TPS40422 and CSD87350

Multiphase Boost Converter power supply designs:

  • PMP2445 – 300W Four Phase Boost for Car Audio Applications using the TPS40090
  • PMP4538 – 500W Four Phase Boost for Car Audio Applications using the TPS40090
  • PMP7850 – Two Phase Synchronous Boost using the LM5122
 
 
PS. For those of you in the US enjoying the Halloween festivities today, check out the wireless powered pumpkin created by Gordon Varney on the Fully charged blog here. Spooky and safe!