SB3a/DB3a Booster Performance

DESCRIPTION/HISTORY
ELECTRICAL SPECIFICATION
OPERATIONAL PERFORMANCE
VOLTAGE PERFORMANCE
ELECTRICAL PERFORMANCE CONCLUSIONS


DESCRIPTION/HISTORY

The SB3a/DB3a incorporates a 5A booster that represents NCE's latest 5A booster design while being offered in a smaller plastic box with a lower price than the PB105 5A booster.  These products are intended to allow one to upgrade the NCE PowerCab from a starter DCC system to a midrange DCC system.

One should NOT confuse the older and discontinued SB3 and DB3 with the SB3a and DB3a that replaced them.  The SB3a and DB3a offer significantly higher performance and reliability.

The SB3a is a combination of a DB3a plus a mid range capability DCC command station in a single box.  A SB3a has the optional ability to be converted to an equivalent DB3a with a software setting change and a flip of a switch on the bottom of the box.

The DB3a is a booster only product.  In theory the DB3a can replace the PB105 but NCE has not stopped making the PB105 booster.  Part of the reason may be coordinated cosmetic design and marketing of the PB105 as the expansion booster for the PowerPro DCC system.   Almost all of the power processing parts used in the design of the SB3a are the same as the PB105.  None the less there are some key part changes too.  Years of experience selling and supporting the PB105 has given NCE real world experience on how to cost reduce the booster's design without any major compromise. 
 

ELECTRICAL SPECIFICATIONS

The electrical specifications for both the DB3a and SB3a are as follows:

1) AC Input Power Requirement:  14-18VAC 50/60Hz, 5A or more amps.
2) DC Input Power Requirement:  18-24VDC, 5A or more amps. (Specification Added with the Manual Dated 1/4/10)
3) Maximum Track Current: 5.1 Amps*.
4) Factory DCC track voltage: 13.8V
5) ABSOLUTE MAXIMUM INPUT VOLTAGE RATINGS: 24VAC or 30VDC.  Exceeding these values by ANY amount at any time will destroy the booster and violate the warrantee of the product.

In the 1st manual dated ??/??/??, there was a statement about needing "cooling" to meet the 5A spec.  This manual did not say anything on how that is to be done.  This statement was later removed in the newer Manual dated 1/4/10.   In other words, no cooling was actually needed.

For information on suitable power supplies for the SB3a, go here: Power Supplies/Xmfrs


OPERATIONAL PERFORMANCE

Unlike the PB105, the 5A current rating of the SB3a/DB3a is not a continuous current rating.  It has a short term duty cycle limit for thermal reasons only.  In other words, as you draw 5A for a several seconds, the booster will shutdown and then recover.  It will appear you have a short circuit but in fact you may not.  Why?  The lack of a metal box used with the SB3a/DB3a has reduced the boosters ability to dissipate the heat required when working with a continuous 5A load.  Unlike the manual indicates, cooling the booster had no effect on the duty cycle.  This observation indicates that that the thermal overload protection is timer based as opposed to temperature based.  However, in actual operational practice connected to a layout, this short term duty cycle limit should not be a problem because the loads on a real layout are not continuous.  The current load is a pulsed load given DCC decoders use PWM motor control techniques.   In other words, this performance limit is something that only bench testing can reveal.   


VOLTAGE PERFORMANCE

Below is the Voltage performance graph of the SB3a/DB3 with a 5A load.

The graph plots the following parameters against the DC input voltage:
1) Switcher Input voltage in DCV. 
2) Output (DCC) voltage (AC)
3) Switcher Voltage drop in DCV.
4) H-Bridge Voltage drop in V.

Both the DC input voltage and the DCC output voltage are measured values at the terminals of the box.  The switcher voltage itself is a parameter only accessible inside the booster.  

The two voltage drops parameters are nothing more than mathematic information based on the measured parameters.

DC input voltage was used since it allows a very precise measurement that can be maintained easily under all load conditions.  However, the SB3a/DB3a is NOT rated for use with a DC voltage.  Why? The input rectifiers will get thermally overloaded.  To do the testing with DC, I had to build a heatsink for the Bridge rectifiers.  I do not rec

Measurements were made after the temperature levels stabilized.  The power supply losses are sensitive to operating temperature.

As a reference point, the output voltage of the booster varies from 14.32V at no DCC load current to 13.44V at 5A DCC load current.  That translates to a 13% Voltage load regulation variation.  Note that the majority of the voltage regulation loss is created by the H-Bridge MOSFET itself. [VdropBridge = MOSFET(Rds-on) x 2 MOSFETs x DCC load current].  The switching power supply is not designed to account for this loss.


ELECTRICAL PERFORMANCE CONCLUSIONS

1) The minimum input voltage needed to maintain a constant regulated DCC track voltage at 5A is 19VDC.   Anything less means a unregulated and 1:1 reduction in DCC track voltage.  See sections below regarding AC transformer and DC power recommendations. 

2) Internal switching regulator needs 4.5V of DC headroom to regulate.  That works out to a maximum 77% duty cycle.  In other words, the regulator loses about 25% of the input voltage when operating in drop out mode at 5A.

3) The DC to DCC H-Bridge voltage drop is relatively constant at 1.2V at 5A.

Test Equipment:
Fluke 8050A DVM's for DC voltages. Tektronix's 540 digital scope for DCC voltage and DCC current.  DCC current measured with a Tektronix DC-50MHz current probe.  H.P. DC Lab Power supplies used.  Kikusui PLZ300 Electronic load in Constant Current mode in combination with a 40Amp Full Bridge Schottky rectifier in front for booster load.