Q5Industrial Electronics

(a) Buck-Boost vs Boost Converter

A boost converter is a DC-DC switching converter topology that always steps up the input DC voltage, producing an output voltage magnitude greater than (or equal to) the input, with the same output polarity as the input. It operates by using an inductor charged from the input source during the switch's on-time, then discharging this stored inductor energy into the output (in series with the input) during the off-time via a diode, with the average output voltage related to input voltage and duty cycle D by Vo=Vin/(1-D) — since (1-D)<1 for any D between 0 and 1, Vo is always greater than Vin. Typical boost converter applications include battery-powered portable equipment requiring a higher regulated voltage than the battery itself provides, power-factor-correction front-end stages in AC-DC power supplies, and solar PV maximum-power-point-tracking (MPPT) converters stepping up a lower panel voltage to match a higher DC bus voltage.

Continuous vs Discontinuous Conduction Mode

Both the boost and buck-boost converters (and DC-DC converters generally) can operate in either of two distinct conduction modes depending on the inductor sizing, switching frequency, and load current. In continuous conduction mode (CCM), the inductor current never falls to zero during the switching cycle, remaining always positive (ripple superimposed on a non-zero average value) — this is generally the preferred mode for higher-power applications since it results in lower peak device currents for a given average power throughput, and the standard idealized voltage-transfer relationships (Vo=Vin/(1-D) for boost, Vo=-Vin·D/(1-D) for buck-boost) are derived assuming CCM operation. In discontinuous conduction mode (DCM), occurring at light load or with insufficiently large inductance, the inductor current falls to and remains at zero for a portion of each switching cycle before the next on-interval begins — DCM operation causes the output-voltage-to-duty-cycle relationship to become load-dependent (unlike the load-independent CCM relationship), complicating closed-loop voltage regulation, but also offers the practical advantage of inherently zero-current turn-on for the freewheeling diode (eliminating diode reverse-recovery loss) at the cost of higher peak/rms device currents for the same average power. Practical converter and inductor design must therefore identify the boundary condition between CCM and DCM at the minimum expected load current, ensuring the converter remains in the intended conduction mode across its specified operating range.

A buck-boost converter is a DC-DC converter topology capable of producing an output voltage magnitude either lower or higher than the input, with the output polarity inverted relative to the input. Its average output-to-input voltage relationship is Vo=-Vin×D/(1-D), meaning for duty cycle D<0.5 the output magnitude is less than the input (buck-like operation), while for D>0.5 the output magnitude exceeds the input (boost-like operation), with the negative sign indicating the inherently inverted output polarity characteristic of the standard buck-boost topology. Typical buck-boost converter applications include regulated power supplies where the input DC voltage may vary above or below the desired regulated output level (such as certain battery-powered systems where battery voltage falls below the required output as the battery discharges), and applications specifically requiring an inverted-polarity DC output derived from a single-polarity DC source.

(b) Dual Converter and Firing Angle Relationship

A dual converter consists of two full (fully-controlled) converter bridges connected in anti-parallel (back-to-back) across the same DC load, allowing the combined arrangement to supply DC power to the load with either polarity of output voltage and either direction of load current — in other words, providing genuine four-quadrant operation, since converter 1 can supply positive output voltage/current while converter 2 (kept in an inverting mode) stands ready to reverse the current direction, and vice versa, without requiring any mechanical or contactor-based reversing switchgear.

Simultaneous (circulating-current) operation: in this mode, both converters are triggered simultaneously and continuously, with one converter operating in its rectifying region (delivering power to the load) while the other is simultaneously controlled to operate in its inverting region, such that their average DC output voltages are equal in magnitude but opposite in sign — this ensures no net average voltage difference exists that could drive an uncontrolled, damaging circulating current around the loop formed by the two converters, while still permitting a smaller, deliberately-limited circulating current to flow continuously (which is in fact beneficial, since it keeps both converters in continuous, rather than discontinuous, conduction, and eliminates the switching dead-time/delay otherwise needed when reversing current direction).

Derivation of the firing angle relationship: for a single-phase (or three-phase) full converter, the average output voltage as a function of firing angle α is Vo=Vo,max·cos α, where Vo,max is the maximum output voltage at α=0°. Since converter 1 is set to firing angle α1 and converter 2 to firing angle α2, and the requirement for the circulating-current mode is that the two converters' average output voltages are equal and opposite:

which directly gives the required relationship between the two firing angles:

This relationship must be maintained continuously (dynamically adjusting both α1 and α2 together, typically via a single common control signal with an appropriate offset/inversion applied to derive α2 from α1) throughout the dual converter's operation, ensuring that whenever the load demands a particular average output voltage (via the appropriate choice of α1), converter 2's firing angle α2 is automatically set to the complementary value 180°-α1, maintaining the zero-average-circulating-voltage condition at all times regardless of the instantaneous operating point demanded by the load.

Non-Simultaneous (Circulating-Current-Free) Dual Converter Operation

As a contrast to the simultaneous (circulating-current) mode described above, a dual converter can alternatively be operated in a non-simultaneous (circulating-current-free) mode, in which only one of the two converters is gated (allowed to conduct) at any given time, while the other converter's gate pulses are completely inhibited (blocked). In this scheme, the firing-angle relationship α1+α2=180° is still used to determine what firing angle the inactive converter would require when it is eventually enabled, but no continuous circulating current is allowed to flow, since the currently-inactive converter's thyristors receive no gate pulses at all and hence remain fully blocked. Whenever the load current direction must reverse, a logic-controlled changeover sequence is used: the presently-active converter's firing angle is first advanced toward the inverting region to force its output current down to zero, a short interlock (dead-time) delay is then allowed to ensure all of the outgoing converter's thyristors have fully turned off and regained forward blocking capability, and only then is the other converter enabled with gate pulses at the appropriate complementary firing angle to take over conduction in the required new current direction. This non-simultaneous scheme eliminates the continuous circulating current and its associated additional converter and transformer loading present in the simultaneous-control scheme, at the cost of a small but finite current-reversal transition delay (during which neither converter conducts) and the added complexity of the current-zero-detection and changeover logic circuitry, representing the classic trade-off between the two dual-converter control philosophies in industrial four-quadrant DC drive applications.