Tries from the outstanding Andrew Trimble and fellow wing Tommy Bowe had put Ulster, runners-up in this competition last year, in command at the interval.
Ulster pushed on to seal their 13th consecutive victory of the campaign with a slick try from full-back Jared Payne and Dan Tuohy's last-minute try, which sealed the bonus point.
The Saints have now suffered back-to-back Heineken Cup defeats.
Jim Mallinder's men will somehow have to recover from this heavy beating and win at Ravenhill in the return fixture next weekend to stand any chance of reaching the knock-out stages.
Northampton captain, Dylan Hartley, could find himself in hot water with the citing officer after he appeared to strike Ulster hooker Rory Best with a forearm or elbow to the face in an off-the-ball scrap.
The Saints' pressing need for a home win, following defeat at Castres in the last round, saw Tom Wood and Courtney Lawes return to action just six days after they helped England beat New Zealand.
Ben Foden and Hartley were also back in action after missing England's autumn campaign through injury - but they and the Saints ran into an Ulster side in unstoppable form.
Saints fly-half Ryan Lamb struggled with the boot all night and he missed an early penalty shot at goal before Ulster took control of the game with an excellent try.
Paddy Jackson's clever chip over the top put Foden in trouble and, when he made little ground with the clearance, Ulster drove the lineout within five metres of the Northampton line.
Ruan Pienaar swung the ball wide to Jackson, who aimed a perfectly-weighted chip towards the corner and Trimble beat Foden to the touchdown.
Lamb and Jackson both missed penalty shots on a cold and breezy night at Franklin's Gardens before the Northampton fly-half finally found his range after a good Saints scrum.
Jackson hit back to re-establish Ulster's five-point lead after Roger Wilson, the former Saints number eight, had claimed an acrobatic lineout and Northampton were penalised at the maul.
Lamb missed again - his third failure in four kicks - before George Pisi turned Ulster on their heels with a kick in behind which Jackson struggled to deal with under pressure from Foden's chase.
Everywhere Northampton looked for attack they hit the white wall of Ulster defence. Ken Pisi thought he had broken clear but spilled the ball in a tackle and George Pisi was flattened by Cave.
Ulster now top Pool Four by six points from Castres with three wins from three. In contrast, Northampton's European hopes may now rest on the Amlin Challenge Cup.
Dan Parks was the driving force behind Connacht's 22-14 victory as they claimed the scalp of French giants Biarritz at the Sportsground.
Parks kicked 17 points, including the conversion of Fetu'u Vainikolo's early try, to seal the province's second win in Pool Three.
Dimitri Yachvili kicked three penalties to leave the visitors 10-9 behind at the break, but Biarritz were decidedly brittle as Connacht's younger forwards outworked them.
Although Imanol Harinordoquy came off the bench to score an injury-time try, Yachvili missed the conversion for a losing bonus point.
It was a tremendous result for Eric Elwood's men, with Parks guiding a hard-running back-line and Mike McCarthy marshalling the young guns up front.
With Gavin Duffy, John Muldoon and Nathan White heading the province's 13-man injury list, Parks skippered Connacht and varied the play superbly early on.
After Yachvili nudged Biarritz ahead, Vainikolo struck for a superbly-taken try that really lifted his team-mates and the vocal home crowd.
David McSharry intercepted a pass from Charles Gimenez near halfway and offloaded for the Tongan winger to charge away for a score against the run of play.
Parks converted to put Connacht 7-3 ahead and the westerners missed out on a second try when Ireland lock McCarthy failed to spot Robbie Henshaw looping around him in plenty of space.
Biarritz looked lacklustre in thought and deed, and tigerish tackling from the likes of Willie Faloon, Michael Kearney and Eoin McKeon helped Connacht keep their defensive line intact.
Biarritz's game descended into niggly phases, with their questionable attitude and temperament resulting in a couple of scraps between the forwards.
They did lift the tempo late on, powerhouse Harinordoquy piling over after O'Halloran was sin-binned during some desperate Connacht defence.
But Connacht deservedly failed to cough up the losing bonus point and they will head to south-west France for next week's return encounter with renewed confidence.
Glasgow saw their Heineken Cup hopes extinguished as they failed to get the bonus point win they needed as Castres won 9-6.
The match was a poor advert for European rugby - one of brutal frontal assaults relieved by long kicks and penalty awards and was little reward for the 3,348 spectators who turned up on a freezing night.
Ultimately it was decided by the boot with three successful penalties from Castres' Rory Kockott outdoing the pair from Glasgow's Scott Wight.
Both teams had had spells of first-half possession but neither goal line had been seriously threatened thanks to determined tackling and referee Wayne Barnes detecting penalty offences.
Glasgow made a livelier start to the second half but after 47 minutes Wight, in a moment which is sure to haunt him, was unable to take advantage of a penalty shot which was straight on from the Castres 22.
In turn Castres twice were unable to make anything of pressure situations on Glasgow's line and lost their advantage by conceding penalties. With 57 minutes gone Kockott was able to nudge Castres into the lead with a long penalty.
The hard-fought exchanges appeared to have resulted in concern for Glasgow captain Al Kellock who went off late on with an arm injury. With Glasgow unable to threaten further they saw their European hopes end as Castres held on to keep their alive.