Hello Patrick,
Thank you for your prompt response to our 24V Step-Up Regulators (U3V50F24) problems. We call them 24V Boosters that we use in our Solenoid Driver Circuit, shown below in Fig 1.
First to answer your questions: “… how much use the problematic units saw before failing as well as how long you have used other units of this regulator for this application in general? “
As part of our ongoing equipment upgrade, we started installing these 24V Boosters beginning in March of this year. To date, we’ve installed 70 Boosters, with five experiencing premature failures after their first trouble-free weeks/months of operation. There were no failures during R&D and manufacturing testing and there were no failures during the first few weeks of operations with many ON/OFF cycles. Currently, we have 65 units running 24/7 with Boosters and we would like to add many more. Therefore, we would appreciate your help to improve our Booster applications.
Our application is illustrated in Fig 1. To start, the +18V Li-Batt is manually plugged into the Vin of the Booster. It charges-up the 4,000 μFd capacitor bank that is directly connected to Vout. Initially, the Solenoid’s MOSFET Sw is OFF and once every day it is turned ON for 1.2 seconds by a microcontroller (not shown). Operations continue 24/7, typically for a month, after which it’s unplugged, maintenance and testing is performed, and then it’s returned to service.
Failures: The five failed Boosters had been working successfully 24/7 during a wide range of weeks and months with many plug/unplug cycles. Typically, the failures occurred immediately when power was again applied to a Booster that had an uncharged capacitor bank load. During start-up the Booster’s peak input and output currents are limited only by the Booster itself. We suspect that damaging start-up transients may cause the Booster to draw excessive current that blows the 3 Amp fuse. We also suspect startup transients may be damaging new Boosters causing them to fail prematurely.
Analysis: So far, I received and tested four of the five failed Boosters. Each failed Boosters’ Vin was opened circuited to GND. Booster #4’s Vout reads 4.6W and the three remaining Boosters Vout’s read less than 1W to GND. (Multimeter test probes +/- & -/+ and wait for steady state).
During initial start with an uncharged capacitor bank, the Booster’s inrush current is limited only by the Booster itself plus what little resistance there is in the wiring and fuse to the +18V supply.
I managed to capture the following Scope photo of a Booster’s Vin startup transient. The yellow trace is Vin (Chan 1) and the purple trace is Vout (Chan 2). Unfortunately, I couldn’t capture my chief suspect, the inrush current, with the equipment I have. The sharp voltage transient at Vin of over +10V then down to about -25 at Vin is surprising since its wired directly to the +18V Li-Batt through the manual plug-in connector and 3A fuse. The steady Vout was not surprising since it is connected to the low ESR 4000 μFd capacitor bank. Sorry the reticule scale didn’t print out.
Scope Photo 1: Start-up transient at Booster’s Vin and Vout
After the Booster survived this abrupt start, it charged the capacitor bank up to +24V and continued running with no-load as shown in Scope Photo 2 below.
Scope Photo 2: Booster running normally at no load with +18Vin and +24V output
Next step: To limit the Booster’s capacitor bank start-up load we’re going to test adding a 10 W resistor in series with the Booster’s output as shown below.
We really like your +24V Step-Up Regulators (U3V50F24) but are concerned we are misusing it because we don’t know its in-rush current and capacitive load limitations.
Therefore we would appreciate your advice.
Best regards,
Tony
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