SEPIC inductors size

[Hammy]

Should SEPIC inductors be sized according to the average load current
or the peak load current?

For instance my load draws one amp pulses (3uS PW) but at ten percent
duty so this is an average of 100Ma. Should the inductors be sized for
1A or 100mA? The minimum load is about ten mA.

Would I be better of designing for DCM or CCM?

Thanks

 

[John Larkin]

On Thu, 02 Oct 2008 13:42:41 -0400, Hammy <spamme@hotmail.com> wrote:

Should SEPIC inductors be sized according to the average load current
or the peak load current?

For instance my load draws one amp pulses (3uS PW) but at ten percent
duty so this is an average of 100Ma. Should the inductors be sized for
1A or 100mA? The minimum load is about ten mA.

Would I be better of designing for DCM or CCM?

Thanks

The switcher loop won't react much in 3 us, so you're going to have to
depend on the output capacitor to supply the load current, and the
switcher basically recharges it between shots. So, size L for the
average, or maybe a bit above, 200 mA maybe.

At 10 mA, it will almost certainly go discontinuous. You may as well
keep it discontinuous at max load, to keep the loop dynamics
relatively constant, but if it transitions tio continuous at some
intermediate load, that's usually OK.

John

 

[Hammy]

On Thu, 02 Oct 2008 12:58:55 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

The switcher loop won't react much in 3 us, so you're going to have to
depend on the output capacitor to supply the load current, and the
switcher basically recharges it between shots. So, size L for the
average, or maybe a bit above, 200 mA maybe.

At 10 mA, it will almost certainly go discontinuous. You may as well
keep it discontinuous at max load, to keep the loop dynamics
relatively constant, but if it transitions tio continuous at some
intermediate load, that's usually OK.

John

Thanks John

 

[Bob Eld]

"Hammy" <spamme@hotmail.com> wrote in message
news:ep1ae4lcn68e696hmk78pu0u8f2qsn1q2v@4ax.com...

Should SEPIC inductors be sized according to the average load current
or the peak load current?

For instance my load draws one amp pulses (3uS PW) but at ten percent
duty so this is an average of 100Ma. Should the inductors be sized for
1A or 100mA? The minimum load is about ten mA.

Would I be better of designing for DCM or CCM?

Thanks

John is right above relative to the load current which mainly comes from the
capacitor. However, you really have to ask the question based on the current
in the inductor, not the load for which you need the duty cycle. At 50% this
current would be 200mA.

The inductor must be sized for the PEAK current in IT for all conditions of
line, load and duty cycle. The inductor current determines the saturation of
the core which under normal operation should be avoided.