Switching Regulator - Part 8

Switching Regulator Selection Parameters:

• Switching frequency
• Inductor value, current ratings and DC resistance
• Filter Capacitor

 Switching frequency

Switching frequency The switching frequency of typical converter ICs on the market is in the range 100 kHz to 2MHz. This leads to the following recommendations:

DESIGN TIP 1: 

Suitable core materials 

Switching frequency < 100 kHz: Iron powder, ferrite, Superflux, WE-PERM 

Switching frequency 100-1000 kHz: Ferrite, Superflux, WE-PERM

Switching frequency > 1000 kHz: Ferrite, WE-PERM

Inductance value 

If there is no application note or software available for the selected PWM controller, inductance can be calculated using the following rule-of-thumb formula:

Buck Converter = > L = (Vinmax - Vout)*(Vout+Vd) /(Vinmax+Vd)*0.3*Iout*f

Boost Convereter => L=(Vout+Vd-Vin min)*Vin*Vin/(2*0.2*Iout*(Vout+Vd)^2*f)

Vout - Desired Output voltage

Vin - Input voltage

Ripple factor - 0.2 to 0.4

DESIGN TIP 2: 

Inductance value

à higher inductance – smaller ripple current 

à lower inductance – higher ripple current The ripple current is essential in determining the core losses. Besides the switching frequency, it is therefore an important parameter for minimising the power loss of the power inductor.

Inductor current ratings

Inductor current ratings The current load for power inductors can be calculated very accurately in terms of DC current load and ripple current load (core losses) using the manufacturers’ simulation software. 

The following approach can be chosen as a rough calculation:

Step-down regulator: Nominal current of the inductor: In = Iout Maximum coil current: Imax = 1.5 x In Step-up regulator: Nominal current of the inductor: In = (Vout/Vin) Iout Maximum coil current: I max = 2 x In

DESIGN TIP 3:

The nominal current for power inductors is usually linked to the specified self-heating with DC current – here self-heating of +40°C is common at the nominal current. According to semiconductor manufacturers‘ recommendations, the saturation current is the point at which the inductance value has fallen by 10%. Unfortunately, this is not a standard value for power inductor data sheet specifications and often leads to misinterpretation among users.

DC resistance:

Once the required values for inductance L and inductor currents are calculated, you select a power inductor with the minimum possible DC resistance. Here the demands are often counteractive: Small size, high energy storage density and low DC resistance.

DESIGN TIP 4: 

DC resistance with the same size 

•  higher inductance – higher DC resistance
• lower inductance – lower DC resistance 
• same inductance for a shielded inductor – lower DC resistance The DC resistance is essential in determining the wire heating losses; this is another important parameter for minimizing the power loss of the power inductor.

Type and EMC

Magnetic shielded power inductors are recommended for EMC-critical applications. The shielding prevents uncontrolled magnetic coupling of the windings with neighbouring conductor tracks or components.

DESIGN TIP 5: 

Use a magnetically shielded power inductor if at all possible. Do not route any conductor tracks under the component and do not place any circuit boards directly above the component, as this could give rise to coupling via the air gap remaining.

DESIGN TIP 6:

Advantage of magnetically shielded inductors of the same type:

à higher AL value, therefore lower DC resistances for the same inductance = lower wire losses. 

Disadvantage of magnetically shielded inductors of the same type:

à slightly increased core losses due to a larger core volume. Given correct dimensioning the core losses remain low.

Output L-C filter An L-C filter at the DC converter output is recommended if a low noise output voltage is required. The components can be selected as follows

DESIGN TIP 7: 

• Select cut-off frequency at 1/10 of the switching regulator frequency 
• Select output capacitor (e.g. 22 µF) 
• Calculate inductance L = 1/ (2 • π • f)^2 • C

DESIGN TIP 8:

Ripple measurements To properly measure ripple on either input or output of a switching regulator, a proper ring in Tipp measurement is required. Standard oscilloscope probes come with a grounding clip, or a long wire with an alligator clip. Unfortunately, for high frequency measurements, this ground clip can pick-up high frequency noise and erroneously inject it into the measured output ripple. The standard evaluation board accommodates a home made version by providing probe points for both the input and output supplies and their respective grounds. This requires the removing of the oscilloscope probe sheath and ground clip from a standard oscilloscope probe and wrapping a non-shielded bus wire around the oscilloscope probe. If there does not happen to be any non shielded bus wire immediately available, the leads from axial resistors will work. By maintaining the shortest possible ground lengths on the oscilloscope probe, true ripple measurements can be obtained.

Source:Digikey AN

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