Electronic – Boost regulator: Using inductors with greater value than datasheet suggests to improve performance

Increasing the value of the inductor will sooner or later turn your circuit into Continuous Conduction Mode (CCM), where there is a constant DC current flowing through the inductor, and the ripple current (the triangle waveform) being superimposed on this base DC current.

This means that the current of the inductor in this mode never goes down to zero. The energy stored in the inductor is indeed \$\dfrac{LI^2}{2}\$, however, only a part of this energy gets transferred in a cycle to the consumer: \${\dfrac{LI_{max}^2}{2}} - {\dfrac{LI_{min}^2}{2}} = \dfrac{L(I_{max}^2-I_{min}^2)}{2}\$, the rest stays in the inductor (more or less permanently). The amount of energy used by the consumer will be the sum of the energy transferred through storage in the inductor, and the energy constantly being transferred by the DC current flowing through the inductor.

The amount of the maximum possible ripple current is determined by the inductance value, the switching frequency, the ON-OFF duty ratio (which more or less equals the output to input voltage ratio), and the voltages connected to the inductor (the input voltage minus the output voltage in the ON phase, and the output voltage in the OFF phase). Increasing the inductance value decreases the maximum possible ripple current. As soon as the output current needed is more than half of the maximum possible ripple current, operation will turn into CCM.

What disadvantages are there for having a larger inductance?

• More turns, therefore higher DC resistance in the inductor, meaning larger copper losses, or having to increase wire thickness/number of strands to compensate for this, thereby increasing inductor size & cost.

• Having a larger inductance, the feedback control loop will be slower, meaning that the power supply will be less flexible adapting to quickly changing loads. This will probably show itself as larger overshoots in response to a unit-step load change.

• When using an asynchronous buck converter, in OFF-state, the free-running diode will be conducting. In DCM, the diode is conducting only as long as the energy stored in the inductor gets fully removed. In CCM, the diode is conducting during the whole OFF-phase, as the inductor current never goes to zero. This means higher losses on the free-running diode - which may be a problem especially because the losses on the FET can be decreased by using a FET with a lower R_dson, but you cannot do the same with the diode. The smaller the output to input voltage ratio is, the more grave this problem could be.

There are two main types of electromagnetic interference. Conducted emissions and radiated emissions.

CONDUCTED EMISSIONS:

The first thing I would try is to look at the output of the boost regulator on an oscilloscope and see how much ripple voltage there is.

If you see lots of ripple then I would add a PI filter at the output of the regulator using a small inductor and two ceramic caps, and a small damping resistor in series with the inductor, where R>SQRT(4L/C). The resistor prevents resonance that could boost the noise at the resonance frequency of the filter 1/(2*pi*SQRT(LC)) Hz.

The cutoff frequency 1/(2*piSQRT(LC)) Hz should be several times lower than the 100kHz frequency of your regulator. Remember that the PI filter is second order, so if you make the cutoff 10x lower (at say 10KHz) you would squash any noise by a factor of 100X.

Also, keep the inductor in your PI filter away from that large toroid or they may couple together which defeats the purpose of the filter.

RADIATED EMISSIONS:

If you think that electromagnetic fields are radiating then steel sheet metal will block both electric and magnetic fields. It needs to be a magnetic type of steel. You basically just need to form it in a box around your noise source (in this case the power supply).