3kW and Smaller Than a Half a Chocolate Bar

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Switching regulator that boosts and bucks

The RBBA3000 from RECOM is a 3kW boost/buck converter in a half-brick package. The boost-buck technology offers and input voltage range of 9V to 60V at a current up to 50A. It is the ideal solution for vehicle on-board networks where there is for instance a second on-board circuit apart from the 12V one. The RBBA3000 is equally suitable for use in battery-powered devices or UPSs (uninterruptable power supplies). In the event of a power failure, it might for example be used to supply a robot from a 48V battery in order to complete the current task and to subsequently return to a safe position.

Its dimensions are 63.2mm x 60.6mm x 13.0mm, with a volume of 50cm³. This compact size is made possible by the exceptionally high efficiency of the converter, reaching 96% along a very flat characteristic over load.

Efficiency over output current
Fig. 1: Efficiency over output current

This results in an excellent efficiency rating even at low loads. At a high power density, the converter needs however to be cooled. This is done by heat dissipation through the soldered or screwed baseplate attached to the bottom of the half brick case. The converter can thus be operated at full power at ambient temperatures up to 85°C. Above this range, derating becomes an issue. At 25°C and 80% load, the MTBF calculated according to Telcordia SR332 is 1, 3.106 hours.

Setting output voltage

In the RBBA3000, the output power Ua can be set with a trim resistor Rtrim, based on the following calculation:

If Rtrim is set, the output voltage can be calculated as follows:

Alternatively, the output voltage can be set by applying a specific voltage to the trim pin. This voltage is determined as Utrim=2,366V – 38,6.10-3Ua. When voltage Utrim is changed, the output voltage of the RBBA3000 is adjusted along the curve shown in  . It is thus possible to regulate and control the output voltage within a range of 0V to 60V, using for instance a microcontroller and a DA, at an output current of up to 50A. The characteristics of these two control variables are shown in Fig. 2.

Setting output voltage
Fig. 2: Setting output voltage

Current limitation

The maximum output current can be limited with a second resistor. It is connected to the Iset pin and is calculated as

The maximum output current can also be controlled by voltage UItrim at the Iset pin. The relevant calculation is

, and the voltage is also adjustable, similar to the voltage settings explained above.

Current measurement

The RBBA3000 comes with a built-in current measuring function, doing away with the common problems associated with current measurement in power electronics circuits. The voltage at the Ishare pin provides the current signal, based on the characteristic shown below:

Current signal based on actual current
Fig. 3: Current signal based on actual current

Parallel circuit

For applications that require more than 50A, two RBBA3000-50 can be operated in parallel. To do this, the two Ishare pins must be connected to ensure equal current distribution to the two converters. In addition, the two converters must of course have the same settings for input voltage, output voltage, sense pins, etc. For details, see the RECOM Application Guide.

EMC

The actual dimensioning of the input and output filters depends on the application, so that we can only consider the worst case here.

In order to meet the standards for conducted and radiated noise, we recommend the following filters for the RBBA3000:

Fig. 4: EMC input and output filters

C1: 470μF/100V, electrolytic capacitor
C2: 10μF/100V, ceramic capacitor
C3: 10μF/100V, ceramic capacitor
C4: 10μF/100V, ceramic capacitor

Differential mode chokeL1: 10μH/56,7A, e.g. Würth Elektronik 7443763540100.

Common mode choke LCM: 1,4mH/50A, Schurter Inc. DKIH-3252-506J-NK

Fig 5 shows the achieved spectrum of connected noise.

Fig 5: Spectrum of connected noise

The relevant volumes are 50cm³ for the RBBA3000, and 52cm³ and 87cm³ for the diff mode choke and the common mode choke respectively. Depending on the actual application, some of the filter elements might of course be omitted.

Applications

We already briefly outlined potential applications of the RBBA3000. Let us now look in more detail at an application where a variable input voltage (20V to 60V) needs to be converted into a regulated output voltage of 12V:

Fig. 6: 12V output voltage from variable input voltage

To achieve a different output voltage, the only component that needs to be changed is the 36kΩ resistor; see also Fig. 2.

Key Data Box:

Key data of the RBBA3000:

Input voltage: 9V to 60V (80V short time)
Output voltage: adjustable from 0V to 60V
Input and output current: 50Amax; adjustable current limitation/regulation
Efficiency: 96%, see Fig. 1
Power dissipation: 47W@Ue = 48V, Ua = 24V, Ia = 50A
Case: half-brick, 63.2mm x 60.6mm x 13.0mm, volume = 50cm³
Cooling: heat dissipation through baseplate, soldered or screwed
Reliability: MTBF according to Telcordia SR332, calculated for 1, 3.106 hours
Comprehensive ambient tests – RECOM sources
Temperature range (with cooling): -40°C to +85°C
Galvanic insulation: input and output grounds connected
RECOM sources: data sheet, application notes, standards.