How to Distinguish Cascading and Mixing of Voltage Conversions

“For functions that require conversion from excessive enter voltages to very low output voltages, there are totally different options. An attention-grabbing instance is switching from 48 V to three.3 V. Such specs usually are not solely frequent in server functions within the info know-how market, but in addition in telecommunication functions.

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For functions that require conversion from excessive enter voltages to very low output voltages, there are totally different options. An attention-grabbing instance is switching from 48 V to three.3 V. Such specs usually are not solely frequent in server functions within the info know-how market, but in addition in telecommunication functions.

If a buck converter (buck) is used for this single conversion step, as proven in Determine 1, the issue of small responsibility cycle happens.

Determine 1. Voltage discount from 48 V to three.3 V in a single conversion step

The responsibility cycle displays the connection between the on time (when the primary swap is on) and the off time (when the primary swap is off). The responsibility cycle of a buck converter is outlined by:

When the enter voltage is 48 V and the output voltage is 3.3 V, the responsibility cycle is about 7%.

Which means that at a switching frequency of 1 MHz (1000 ns per switching interval), the on-time of the Q1 swap is barely 70 ns. Then, the Q1 swap is turned off for 930 ns and Q2 is turned on. For such a circuit, a switching regulator should be chosen that enables a minimal on-time of 70 ns or much less. Selecting such a tool presents one other problem.

Usually, the conversion effectivity of a buck regulator decreases when working at a really small responsibility cycle. It's because the time accessible to retailer vitality within the Inductor may be very quick. Inductors want to offer vitality throughout longer off-times. This normally ends in very excessive peak currents within the circuit. To cut back these currents, the inductance of L1 must be comparatively massive. This is because of the truth that a big voltage distinction is utilized throughout L1 in Determine 1 through the on-time.

On this instance, the voltage throughout the inductor through the on-time is roughly 44.7 V, the voltage on one facet of the swap node is 48 V, and the voltage on the output is 3.3 V. The inductor present is calculated by the next system:

If there's a excessive voltage throughout the inductor, the present within the inductor will rise for a hard and fast time with the identical inductance. With a view to scale back the height inductor present, it's obligatory to decide on a better inductor worth. Nevertheless, increased inductance values ​​improve energy losses. Below these voltage conversion circumstances, Analog Gadgets’ high-efficiency LTM8027 µModule® regulator module achieves solely 80% conversion effectivity at 4 A output present.

At present, a quite common and extra environment friendly circuit answer to extend conversion effectivity is to make the most of an intermediate voltage. Determine 2 exhibits a cascaded setup utilizing two high-efficiency buck regulators. Step one is to transform the 48 V to 12 V, then within the second conversion step this voltage is transformed to three.3 V. The overall conversion effectivity of the LTM8027μModule regulator module exceeds 92% when dropping from 48 V to 12 V. The second conversion step makes use of the LTM4624 to scale back 12 V to three.3 V with a conversion effectivity of 90%. The overall conversion effectivity of this scheme is 83%, which is 3% increased than the direct conversion effectivity in Determine 1.

Determine 2. Voltage from 48 V to three.3 V in Two Steps, Together with a 12 V Intermediate Voltage

This may be fairly stunning since all the ability on the three.3 V output must undergo two separate switching regulator circuits. The circuit proven in Determine 1 is much less environment friendly because of the quick responsibility cycle, which ends up in a excessive peak inductor present.

Along with conversion effectivity, there are various different features to think about when evaluating single-step buck architectures with intermediate bus architectures.

One other answer to this basic downside is the LTC7821, ADI’s new hybrid buck controller, which mixes a cost pump with buck regulation. This ends in an obligation cycle of 2x VIN/VOUT, thus enabling very excessive step-down ratios at very excessive conversion efficiencies.

Determine 3 exhibits the circuit setup for the LTC7821. It's a hybrid synchronous buck controller that mixes a cost pump to halve the enter voltage and a synchronous buck converter in a buck topology. When utilizing it to transform 48 V to 12 V at a switching frequency of 500 kHz, the conversion effectivity exceeds 97%. Different architectures can solely obtain such excessive effectivity at a lot decrease switching frequencies and require bigger inductors.

Determine 3. Circuit Design of Hybrid Buck Converter

4 exterior switching transistors are required. Throughout operation, capacitors C1 and C2 carry out a cost pump operate. The voltage generated on this method is transformed right into a exactly regulated output voltage by a synchronous buck operate. With a view to optimize the EMC traits, the cost pump adopts gentle switching operation.

The mixture of cost pump and buck topology has the next benefits:

benefit

The conversion effectivity may be very excessive because of the optimized mixture of cost pump and synchronous switching regulator. Exterior MOSFETs M2, M3 and M4 are solely low voltage tolerant. The circuit can also be compact. The inductor is smaller and cheaper than the only stage converter strategy. For this hybrid controller, the responsibility cycle of switches M1 and M3 is D = 2×VOUT/VIN. For M2 and M4, the responsibility cycle is D = (VIN